It is an audible and almost tangible reaction to what is going on. If we hear the girl moaning we know (or at least think) she is enjoying what we are doing, and we feel good knowing that we are pleasuring our partner.
I do not think so. I am not all that dominant in bed (don't get me wrong when the time is right I can be, it's just not my default setting if you will), but I definitely enjoy knowing that my partner is being pleasured. If there is no indication to me that my partner is enjoying the sex I struggle to enjoy the sex (at those times I typically take over and make sure she orgasms a few times before continue having sex; that almost always helps).
It's probably biological. To be as simple about it, it makes us become more sexually aroused for whatever reason. Guys might try to rationalize it in different ways, but it's really just our biology that makes us that way.
It because it a sing of passing that let the person that you are with that you are in to what they are doing in the bed room both men and woman do it so don,t think guys are the only ones what to moan in the bed room :)