Can I keep on wearing my braces after the treatment? Or do I have to wear a retainer now to retain the position of my teeth?

I've been wearing my braces for nearly a year now and my top set of teeth are already aligned so my dentist wants to remove them and use retainers. Cant I just wear my braces forever? Does it not align my teeth or make them stay in place when I use braces? Because I see a lot of people who wear braces even tho their teeth is aligned