"Ultimate purpose" Have you ever heard the word fun? Thats why most sex exist, there is even a list of things called contraceptives.
That said, she being pregnant is possible, she is 100% going to get a lot of creampies, but then you remember that, she is working (And drinking a lot) in a bar, and is all fun until the little one is supposed to be born and the dev needs to think what to do with it, thats why pregnancy is usually put like a kind of ending more than a mechanic in most games.
The implementation as a mechanic is annoying, just like pregnancy in real life.