no, YOU don't need it to enjoy the game, but it is a useful tool to have, for example DEX in general adds to your AC, the more DEX you have the More AC you'll have as a result, which will help you survive longer in D&D.
Eh kind of...mostly light and spell based armor benefits from DEX. Medium armor can get up to a max of +2 from DEX so limited usefulness there and heavy armor gets no dex bonus at all. At any rate I wouldn't try to force DEX if it that's not what your class is about.
