The sex is much better if she is excited and even more so if she is having orgasms. The pussy gets wetter and tighter. Hell, one lady I saw had orgasms so tight she would push your cock out of her pussy or ass when she came if you were not rock hard and deep inside her. And she could cum a lot in an hour; some of the best sex I've ever had.
So,selfishly, I like it when my partner is excited and into the sex, so her pleasure leads directly to increased pleasure for me.