What do mission organisations do for these people?
This is a hard question to answer.
I guess there are different ways to look at it: missionaries say they're bringing the Gospel and helping the tribal people have a smoother transition into this constantly shrinking and technologically developing world; others say that the missionaries are butting into these people's lives and destroying their culture.
I say that it's somewhat of a hybrid between both. I do understand that when missionaries go to these remote places, they are risking their lives to bring the Truth to these people, and I really do respect them for doing this, because, to be totally honest, if someone asked me, "Would you like to share the Gospel with cannibals and head-hunters?" I'd say "Uh...let me think about that. Nope. I don't associate with people who see me as an Asian appetizer and literally want my head on a platter." At the same time, I think it's difficult for missionaries to introduce something so life-changing without altering a culture. I won't say destroy, because most times, even after a majority of a tribe has converted to Christianity, they keep many of their traditions and way of life. I say most because this hasn't always been the case, as some missionaries get carried away and think the best way to approach a different people group is by becoming tyrants, like the missionary in James Michener's novel Hawaii that Don Richardson mentioned in Do Missionaries Destroy Culture? Most of the time, though, it's not like this. Missionaries do truly care about bringing the Gospel to these people and changing their lives for the better.
On the other hand, though, some part of a tribe's culture are changed, but not obliterated. Obviously the religious aspect of the group's culture changes, but just because the people become Christians does not mean they can't eat the food they used to eat and celebrate their old holidays, so long as they're not pagan holidays anymore, I guess. For example, I'm a Korean that's a Christian, but that doesn't mean I can't celebrate 추석 (Chusok: a Korean type of Thanksgiving) and 설날 (Sulnal: the Chinese New Year).
In the end, it's all about the intention of the missionaries who make contact with the distant people and how they choose to approach them. If a missionary truly cares about the people and doesn't mean to totally change a tribe's way of living, it's all for the better, because, like Don Richardson said, it's not like these people are totally going to be isolated for long in this world of competitive entrepreneurs, who would most definitely destroy a group's culture by exploiting them and all that they have.