You apply different amount of breaking. Sometimes you would only use the rear break (for example, if you want to take a corner by locking and sliding your back

You apply different amount of breaking. Sometimes you would only use the rear break (for example, if you want to take a corner by locking and sliding your back wheel). To illustrate this point further: the common setup is to have the rear break on the right hand side of the handlebar. But some people ride it the other way around. If you are used to one way and then have to ride the bike that is set the other way (say, a borrowed bike), it completely messes up your riding.