Reporting rape is a very brave thing to do, and it's unfortunate that we have to be asked if we're doing it for vengeance or "to get back at him".. but it's because so many women report rape only after the guy does something like cheating on the girl, for example. Asking if it's for vengeance is to help weed out the false accusations (cops told me this when I was reporting my first rape).
After reporting it, a detective calls you. Sometimes its for him to set up an interview with you, and sometimes it's to set up a "trap" for the guy, where the detective has a phone with a tape recorder attached to it, and they record the call that YOU make to try to get the guy to admit what he did. After that phone conversation, you go home, and the detective calls the guy to tell him he's being accused of rape and the detective "interviews" him over the phone, basically.
I dropped the case within a week, because I didn't want to reopen that wound and fight the whole thing, when the chances of him actually getting jail time were slim to none. It wasn't worth it to me. But I did feel good about the fact that I reported it, and it's on his record in case he does it again. It's not easy, but it can be harder to deal with the feelings of "what if I reported it, what if something could have happened?". You never know what could already be on his record. He might have done this to someone else who put it on his record.
Just because you put the complaint on his record, that doesn't mean you have to press charges.
I chose not to report the second guy, because he lives a stone's throw away from me and knows where I live, and I felt like it would be dangerous to report him (pissing him off more). But, I got the rape kit done in case I change my mind.
Hope that's helpful.