Combos are a way of maximising the minimum amount of cash a consumer will spend. Most food joints, movie theatres, etc. so cash in on combos.

Let me give you an example:
I went to one of those huge new theatres. Maybe it was a Silvercity. Anyhow, as with many other people, I often like to get popcorn and a drink when I go to see a movie. But, not being very hungry and knowing the high price of movie theatre food, I decided to get a small popcorn and a small Coke. Here's basically what happened:

ME:    Can I get a small popcorn and a small Coke, please?
THEM:  I'm sorry, sir. We don't have small sizes.
ME:    Uh.. well.. I guess I'll get a medium Coke and a medium popcorn then, please.
THEM:  Uh, sir.. for 25 cents more you can get our Extra Value Combo.
ME:    What is that? And how much is it?
THEM:  For $9.50 you get a large popcorn, a large Coke and a Mars bar. PLUS the popcorn and Coke are refillable.

So you're led to believe that the best 'option' is $9.50 for more food than you want. When, really, the best deal is to buy food ahead of time and sneak it into the theatre.

Another good example:
Pizza Pizza. Buy one pizza, get the next one for only a buck! But how much is one pizza? It's $13 & fucking 99¢.Buy two pizzas or spend way too much!

This isn't really a new concept. I mean, most places have always rewarded customers who buy more stuff. You buy more stuff, you get a better deal. But.. combos seem to be a little more sinister. You see, when combos are present, if you buy too little, you get punished.

A combo is also a combination of moves executed in rapid succession or a combination of button presses/joystick movements in video games of the fighting genre.