Why do some braces have a loop in them?

It is used to help distribute the pressure of the wires to turn a tooth or to help hold it in place. They also do it that way to make the process go faster.

Actually that loop is in the archwire so that when you get rubber bands on your braces thats where they hook onto. But sometimes only certain loops are used and not all of them. It doesnt really have anything to do with pressure. Hop that helps

Those loops can be adjusted to give the arch wire more spring so it creates force to move the teeth. There are also some loops to hook rubber bands to, but I don't think that's what you aer describing here.

That is how they tie the braces to the wire

If it is at the bottom and top then it's probably to place a rubber band to help correct overbite or underbite. (trust me, I do it every night.)

on the back of mine, its a loop called a buccaneer, and it holds the wires.

Just so you can have a second opinion, I concur with Dr. Sam.