Answer :

it teaches you to work independently and to do things at whatever moment needed. say if you forgot a line at a actual performance, it would help you come up with something so theres no delays in dialog
Improv is the art in drama of making things up.

It is important in Drama because it gives you kind of an insight in how you have to think fast to doing a skit. It helps get your brain to react faster to a question, or and act. It may also help you be actually funny XD.