Andrew!

Dont forget the butt cheek and knee tape for those really wide (wo/)man-eating off width cracks!
Generally:
1 Tape on the hands and sometimes fingers is often used to protect the skin when crack climbing (hand taping) or climbing sharp incut holds (fingers). Tape can also help prevent callousing during extended gym sessions (callouses are scar tissue which is weak and rips open easily =flappers=OW!)
2 Taping the fingers, wrists and elbows is done to protect weak or injured tendons. Im not convinced elbow taping works at all, at least it didnt seem to help me. Only use tape for fingers and wrists when recovering strength after injury or when pushing much harder grades than usual on tweaky small holds.
Habitual taping is, as Justin points out, a bad idea. The fingers lose strength and stability and become dependant on the tape.