Just to add to this, simplified forms are mostly confined to Joyo kanji. The old forms do, however, exert their existence even in modern kanji as sub-pieces of (mostly rarer) other kanji---e.g. 専 used to be written 專, which still exists as the right half of 慱.

Also, there are lots of cases where the "simple" modern form doesn't really seem all that much simpler when viewing it in a typeface. My ad-hoc hypothesis is that these simplifications were considered with handrwiting in mind, e.g. 戻 (modern) vs. 戾 (original).