Question in title. In a situation like this, I could see the tribals having (unknowingly) the most to gain, but not really having anything the advanced civilization would need, and ships going back and forth would raise the cost of goods that would be cheaper produced in the homeland.
Any ideas?
Read History
History has already seen this scenario play out. Just because a culture is less technologically advanced doesn't mean they won't have things the more advanced culture wants. In the colonial era native Americans traded everything from land to beaver pelts to wild horses to the Europeans, whom made a very tidy profit flipping said items at huge mark ups. A ship arriving in Europe from the east indies filled with cloves and nutmeg used to be worth literally twice its weight in gold. Exotic fabrics, spices, raw materials, artwork, exotic pets, the list of things a more primitive culture might have that the more advanced one would desire is endless. It is mostly dependent upon what the more advanced culture considers valuable, and things don't have to be valued for a logical reason. Maybe a xenophilic fad strikes the more advanced culture and suddenly ooga-boogan fertility idols are all the rage among the wealthy elite and their resale value skyrockets for the duration of the fad.
The other answers give good solutions, but I would like to suggest one more:
For political reasons
The romans traded the germans and even sent gifts to them. This was done in order to excercise political/diplomatic infuence among them. The goodwill of the Popolus Romanus could lift up a local chieftain by the gift/trade of weapons and jewellery and even luxories like glassware, giving him means to reward his followers much more richly than unfriendly chieftains could. Peace could be held up for decades this way without needing to mobilize the legions from their permanent camps. (which was always a costly and risky operation)
In a similar way, if the advanced civilization can not completly kill off the tribals (or does not want, being enlightened), they can by carefully choosing the partners they cooperate with, infuence them to their own purposes.
Looking to history, there are a variety of good reasons.
The locals have a much better grasp of the land than the newcomer high-tech folks. So rather than waste their limited resources mapping out the territory in detail, they just trade with the natives for whatever resources they need. Especially in the short term period when high tech people first arrive in a new territory, it is valuable to trade with the locals who know where food, dangerous predators, and/or local herbs with medicinal value can be found.
Maybe the high tech traders are short on person-power. So they basically outsource their labor to the locals. It may be cheaper or easier to let them do the grunt work of hunting and farming (or any form of manual labor), while you focus on mining or building a city or some other "great task."
If the thing you are trading for is dangerous to acquire, perhaps it is to your advantage to pay the low-tech folks to risk their lives instead of yours.
Trade can help bridge the two civilizations together. Wars and violence are less likely if both sides have some kind of profit to be had by not attacking the other side.
This all comes down to basic economic theory. Supply and Demand. Any situation that results in a surplus on one side and a demand on the other side, there will be trade. This could be because the high tech side places a high price on (say) art or fabrics or goods produced by the low tech side. Or the low tech side sees great value in trading for the better quality tools or goods from the high tech side.
Geographical biophysical adaptations, or mutations, have caused the indigenous people of Tribe X to produce a certain anitbody in their blood that has yet to be understood or reproduced in any medical facility.
The tribe is vehemently against the desecration of body except in homage or tribute. So, periodically, the mainland people orchestrate trades with the tribal people that enrich their lands, fortify their structures, and provide spices otherwise unobtainable to the Tribe X lands. But nothing outside of natural materials or the tribes would reject it. They do not want the pollutions of the rest of the world to interfere with their way of life, but they do realize stronger structures and certain minerals allow for survival in harsh weather or seasons where fragile crops would otherwise be threatened. So, of course, like all human existence, a little corruption comes into place and some chief somewhere decided that a fairly regular ritual to "the gods" grants their people a leeway in their beliefs such that they are no longer condemned for trading a quart of blood on a monthly basis in exchange for the goods used to enrich lands and fortify homes. You know, the things people used to do death sacrifices for except in this case you get actual evidence instead of insane beliefs.
This way your tribe remains in tact. They cannot be removed or killed because medical experts have yet to understand their mutation or whether or not it is wholly regional, regional and biological, etc. The best they can do right now is allow the tribe to continue uninterrupted and occasionally bring supplies in exchange for the next batch of antibody rich blood.
There's plenty of historical records of human structures having only one primary export. In this case, it's blood. But that doesn't mean there's no demand for tribal arts and housewares. Maybe they make a mean cooking kiln too.
Eventually medicine may have all the answers and realize they dont need the tribe anymore or just outright kill them and take their land when the answers are found. But hey, nothing new right?
Maybe this advanced civilization feels it has more important things to do. It would be good if you told us how advanced they are, and how primitive the tribals are.
They could have the tribals work in factories building products like communication devices and computer chips. They would be extremely cheap, since you don’t have to pay the tribals, just give them a fancy doodad here and there.
Get in good with the tribal oligarchs and you can make sure they stay totally loyal. You could send a secret operative to go secretly infecting the chief's kids with a lethal but curable disease, and then give them a cure to make an even better impression.
You could give them some new advanced infrastructure as well, like roads, sewers and communications, pushing them into an Industrial Age, and making them better workers.
Eventually, if they had resources you needed, you could just have them trade with you normally
