There are many reasons alien races would interact with primitives.
Morality - they might be utilitarian and desire to relieve the suffering of other sentient beings. Or force their weird alien morals onto them.
Competition - we might not be a threat now, but given a sufficiently long time, we could evolve into one. If they really don't care about us, it would cost them very little to exterminate us early.
I see these mentioned a lot, and sci-fi is big on the latter. The one I rarely see come up is "entertainment."
If we were traveling amongst the stars today, with our values and morality as they currently are, and happened upon a world of what we'd consider to be primitive life, we'd probably scoop some up and put them in a zoo, or broadcast them on TV. We'd want to study them, learn what makes them tick, maybe try to see if communication was possible, but in the end, we'd get entertainment value from them.
Even if we left them be and studied from a distance, the human race would be fascinated. New life! A new place! We like new things. The news would cover these creatures ("Are they a threat to humanity? News at 11."), they'd be woven into stories and shows and movies in one form or another. If we could safely bring them back, people would want to see them in person.
We'd get a lot of entertainment value out of these new creatures. We're a curious race who manages to get bored of spectacular things quickly, which is a large reason we continue to innovate and push forward.
There's no way to know for sure what would motive another intelligent race to advance to the point where they'd reach for the stars, but if any of them shared a degree of curiosity, I doubt they'd pass us up if they found us. We'd be something new for them.
Maybe we'd end up in their stories, their drawings, their videos, or even their zoos. Maybe they'd be interested in how we smell, or the way we dress and decorate ourselves, or the things that we watch on TV. Our languages, our fights, our culture. Or maybe just the way we go about our business, missing some big cosmological truth that's so obvious to them, as if we're adorable little children.
Maybe we'd provide just enough entertainment value to be worth a quick trip. Or a long stay.
If we discover intelligent alien civilization, there'll be a massive effort put into simply tapping into their culture. Their internet, TV, magazines, history, everything we can our hands on. The sheer volume of cross-cultural exchange which would be suddenly and immediately valuable would be enormous.
This assumes we'd recognise the interaction as an interaction.
A super-intelligent species would have no trouble modelling and manipulating human politics from behind the scenes.
It's naive to assume that an interaction will look like the old explorer or conquistador model - a ship arrives, someone gets off it, and then demands to talk to the chieftain, or starts killing people and blowing shit up to make a point about power.
The set of all possible - and effective - interactions which doesn't look like that is much bigger than the set that does.
That's a very forced explanation by science fiction writers to justify it. I wouldn't expect actual alien civilizations to invent such an arbitrary rule.
Morality - they might be utilitarian and desire to relieve the suffering of other sentient beings. Or force their weird alien morals onto them.
Competition - we might not be a threat now, but given a sufficiently long time, we could evolve into one. If they really don't care about us, it would cost them very little to exterminate us early.