A good controller is a lazy controller.
You do the minimum amount of work necessary to get them all in the right place.
-overheard in a conversation between ATC instructors
Air traffic controllers are a real lazy bunch. They rarely do more work than absolutely necessary to get the job done. Their ambition is to control no aircraft. An empty strip board is a home run. They have one of the best 'pay/actual hours worked' ratios this side of celebrity Board members. Anything that can reduce their workload is a good thing; separation permitting, more planes can be crammed into the same patch of sky or they don't have to do as much work, depending on who you ask.
Standing agreements are implemented in the spirit of controller apathy. They concern how flights to a certain destination are “presented” to a sector of airspace. Continuing said apathy, I will use my former training airspace to help explain how this works, rather than take any time to come up with an interesting sector of airspace of my own.
Our fictional sector of airspace, which we'll call 'Stackers', sits roughly centrally on the south coast of England; ideally located between the glut of busy London airports (Heathrow, Gatwick, Stansted, Luton, what have you) and the southern tip of Britain, right where a large proportion of the traffic from the Atlantic enters and exits the country. It also sits south of Birmingham, Liverpool and Manchester airports, all quite busy sections of UK airspace.
In a location like this, but to a degree in any location, we're going to get some repetition of flight destinations. A large proportion of the eastbound traffic coming in from the Atlantic is bound for London airports (Heathrow, Gatwick, Stansted, Luton, what have you). A large proportion of northbound traffic through the sector is going to land at Manchester or Birmingham.
I forgot to mention something about ATC that's useful in understanding the motivation for standing agreements: coordination. The UK Civil Aviation Authority's Manual of Air Traffic Services (MATS) Part 1 defines coordination as: the act of negotiation between two or more parties, each vested with authority to make executive decisions appropriate to the task being discharged.
Controllers have to coordinate everything, and it's a right ball-ache.
A country's airspace is divided up into a number of 'sectors'. These exist so that the traffic can be divided up between several controllers, each with clear delineations of responsibility. It is common for an aircraft flying between two places to pass through two or more sectors, and therefore to be "worked" by several controllers.
When you, as a controller, send an aircraft from your sector to another, you don't just give the pilot the radio frequency to call and say “bye-bye.” You have to warn the next sector that the flight is coming, and come to an agreement with them about how you will "present" it: that is, the aircraft's altitude, the approximate time it will enter the next sector and its heading (if applicable).
Such a warning usually takes the form of a phone call. Commonly, a person separate from the controller makes these calls; this person is referred to as the 'Planner' or 'Coordinator' (the controller, who actually talks to the aircraft has the 'Tactical' position). Now, if you're lucky and the next sector already has details on the flight (which they should, if the aircraft has a flight plan) then you just phone up the next sector, tell them the time the aircraft is estimated to enter their sector and the altitude you want to give it to them at:
“Hello, Block sector.”
“Hello, Stackers here, requesting coordination on Air France 670.”
“Air France 670: I have the details, pass your message.”
“Air France 670, estimating 'NODER' at four-zero, flight level 240.”
“Roger, Air France 670, flight level 240 is coordinated, contact Stackers on 132.3.”
That's an ideal coordination. The next sector accepts the aircraft as-is, and you can get rid of it. But it's not uncommon for the next sector to be unable to accept the aircraft as 'offered'. Re-running the example above, the next sector could request a different level:
“Air France 670, estimating 'NODER' at time four-zero, flight level 240.”
“Air France 670, unable to accept at 240, I could take him at 260 or 280?”
“Okay, I'll give him to you at 280.”
“Roger, Air France 670, flight level 280 is coordinated, contact Stacker 132.3.”
...or could request the aircraft is positioned using headings:
“Air France 670, I can accept him at 240 on a heading on the west side of the airway.”
“Roger, 240 on a heading on the west side of the airway.”
“Okay, Air France 670, flight level 240 is coordinated, contact Stacker 132.3.”
There are many minor variations on this, but you get the idea. This process has to be run for every aircraft going from one sector to another.
Except that it doesn't.
There are these magic things called 'standing agreements'. Once upon a time, someone looked at a breakdown of Block sector's traffic and thought: “Hey, look how many flights we have going through our sector for Heathrow every day. We usually transfer them at more or less the same level; why couldn't we get together with Stacker and agree a specific level we would give them all Heathrow traffic at? If we did that we wouldn't need to make phone calls to coordinate any of them, and I would get more time with page 3.”
And so they did. The CAA does in fact define standing agreements as one of the acceptable forms of coordination in air traffic control.
A standing agreement is an agreement between two sectors about how they will present certain flights to each other. Telephone coordination only needs to be done if a flight cannot be presented in the agreed manner for whatever reason, or if the conditions of the standing agreement do not apply to a particular flight.
For example, Stacker sector has an agreement with Block sector about flights going to several London airports. Stacker sector will send all such flights to Block sector at flight level 250; telephone coordination is only necessary if this agreed level cannot be achieved for whatever reason.
If there is a situation whereby two aircraft going to the same destination are arriving at the same point at around the same time, they cannot nominally be put at the same level per the standing agreement, but they could be put on parallel headings five miles apart and a phone call made to the sector concerned:
"Stacker sector?"
"Yeah, Block sector. Air France 670 and Speedbird 2 are coming to you on parallel headings at 250."
"Air France 670 and Speedbird 2 on parallel headings, roger. Thanks."
Standing agreements may also specify aircraft positioning: for example, that aircraft cross from one sector to another on a particular side of an airway (a logical 'corridor' in the sky used for navigation). A complimentary agreement will likely be in place for aircraft heading in the opposite direction.
It is not uncommon for standing agreements to have conditions or allowances attached; an agreement might state that once an aircraft has been transferred to a particular sector, that sector may immediately issue instructions to that aircraft - climb, descent or turns - even if it has not actually reached their airspace yet. Bear in mind it is also not uncommon for an aircraft to be transferred up to about thirty miles before it has left a particular sector, because the present sector needs to do nothing else with it and because controllers can usually see twenty or thirty miles beyond the bounds of their sector). These provisions can be quite detailed; an agreement may allow turns to the right but not to the left; descents but not climbs.
If an aircraft does not 'qualify' for the conditions of a standing agreement it must still be individually coordinated. Controllers still do a bit of work here and there.