For most
plumbing jobs, you obviously need the
water shut off.
If you're going to disassemble a
valve, perhaps to
replace a washer, you can't have water
pressure behind that valve.
If you're going to fit new pipe, whether it's
soldered copper pipe or (chemically) welded
plastic pipe, the
joints must be perfectly dry as you make them.
And so on.
And so that you can do this, all but the most ineptly-installed piping systems include shutoff valves so that you can shut the water off when you need to.
But sometimes those valves are missing, and even when they're there,
all too often they don't work, leading to a kind of infinite regress, chicken-and-egg problem: how can you get the water shut off so that you can fix the broken shutoff valve
(so that you can then do the job you started to do)?
Necessity being the mother of invention, there are various tricks for stopping the last trickle of water that's sneaking past a faulty shutoff valve, or for getting the water shut off without a proper shutoff valve at all.
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You can stuff a piece of bread into the pipe the water is trickling out of, and quickly make your joint.
When you turn the main valve back on,
you can wash the now-soggy bread pulp out of the system through a nearby tap.
(I've never actually tried this, but plenty of people swear by it.)
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Years ago I saw a device called a "Dutchman's Thumb" described in the "What's New" column of Popular Science.
I've never seen one for sale, but I made one once:
It's an internally-expanding doohickey that you insert into the pipe that the water is trickling out of.
Now, this doesn't do you much good if you wanted to attach new fittings to that pipe, because you can't very well do that with the handle of the Dutchman's Thumb sticking out, so the second-order trick is that you insert the Dutchman's Thumb through the long axis of a tee with a side threaded outlet, then attach the tee, then remove the Dutchman's Thumb.
The water now trickles out through the side threaded outlet, and you can make the rest of your joints on the downstream side of the tee in perfect dryness.
When you're all done, you put a threaded plug in the side outlet, since a threaded joint is the one kind you can make while wet.
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When there's no shutoff valve at all, not even at the water meter out at the main in the street, there are devices which real plumbers use which consist of an insulating bag to be wrapped around a section of pipe, along with a source of Extreme Cold such as decompressing CO2 or liquid nitrogen.
You freeze the water in a short length of pipe (short enough that the pressure doesn't build up and burst the pipe)
and the ice plug serves as your shutoff.