Yes, there are reasons to sleep with the same person, without it meaning you have feelings for them.
The more partners she has, the more chance that one of them has a STI or a STD, and safer sex or not, the higher the chance that she gets one (and possibly passes it onto you).
The more partners she has, the more times she has to go through the first reveal, reveal of herself (showing herself naked and vulnerable in front of someone for the first time) and reveal of the other person (will she like what she sees?)
With every new partner also comes a new way to have sex, and the first time is often mostly about fumbling as you learn each other's "Style". Having only one-night stands would mean never going past the fumbling stage with anyone, never having sex with someone who already knows what she likes and that she's comfortable with.
On top of that there is of course the fear that any stranger could be violent, crazy, obsessive, so once she has someone she trusts who can do the job, looking for a different person might seem like an unnecessary risk.
Does having the same partner over and over again increase the odds that she'll fall in love? Sure, although there is no way to predict that. But it reduces the odds she'll catch something, get physically hurt or even killed, have terrible experience after terrible experience, etc.
It's a choice that makes sense, so yes it is "normal" since that's what you asked. Preferring one night stands would also be "normal", it would just mean different priorities is all.