I don't have one are the really necessary?
In my opinion yes. As soon as you do workout with a lot of movement and faster movement it will be easy more comfortable. If you do only stretching or yoga workouts it might not be that useful but in general i would say!!
Yes! They keep your boobs where they should be and save your regular bras from getting stretched out from movement/ruined by all that sweating. I'd recommend trying one on before purchasing, because it needs to fit snugly but comfortably or it won't do any good. Also, make sure it has enough support for the workouts you do.