There are numerous reasons to get braces. If you have teeth that do not match right with the bottom, you might have a bite issue that braces can correct. Getting braces doesn't mean you will have to have teeth pulled. In the past, orthodontists pulled pre-molars to make space for crowding. This is still done today, but much less often (especially for teens). Instead, if your jaw is still growing, an orthodontists can expand you jaw to correct crowding. In adults, where jaws have stopped growing, orthodintists can file the side of your teeth done slightly to correct crowding.
Often times an orthodontists will ask you to have your wisdom teeth removed. This is done b/c it easier to place the appliances, wisdom teeth easily crowd your other teeth, etc.
I am sure there are exceptions to all the example I gave. Your best bet is to go visit a good orthodontist. Hope this helped!