Most of time its just to keep up a client relationship or to get their dose of attention. However if she contacts you a lot, and you've seen her several times and things get better, there is no harm in asking her out but be prepared for rejection while being cool about it because you don't want rejection to ruin your relationship.
I've had many girls contact me as well, I even had girls ask me out. One girl in particular, im sure she knows herself if she reads this, we were both very into each other, but I chickened out because I had previously gotten out of a relationship and I couldn't see myself sharing my gf. (sorry AR i still think of you)
something to realize. When you are in this hobby you are paying for your fantasy, and the rule of this hobby is to NOT "fall into the fantasy" completely. If you're asking an SP out, keep both hands on reality because she is. By this I mean, dating an SP is not what most people think.
I constantly see people in a never ending cycle of them thinking they are special and unique in this hobby. Yeah no... most probably not.
good luck