Well, 214 basically gives you quick and easy access to ws and full crouch moves (ws meaning while standing or while rising). For example, 214~B gives you wsB and 214~3aB fould give you FC 3aB. It also acts as a small backdash and tech crouch, but mainly you'll be using it to bust out Sets' awesome ws/FC game.
236 is a front dash that allows easy access to 8 way run moves. For example, 236~3B performs a standard 33B. Like 214, it also has some evasive tech crouch properties. It kind of adds a little bit of evasion before whatever move you were planning on doing. Also, it adds a little extra distance to any 8 way run move since the animation is a forward dash.
Both of these moves also allow you easy access to sets of moves in situations where it would be difficult to perform them otherwise. Instant ws moves, for example, are much harder to do than a simple 214 (which serves the same purpose).