Herman Lehmann was kidnapped from his farm by Apache Indians in 1870 when he was 10 years old and then raised and assimilated into their tribe. I don't understand why the Apache did this. What purpose did it serve them? Why would they want another child to look after and feed, especially a white one? It's not like they killed his family and took pity on him either. They seem to have just grabbed him and his brother when they were alone and taken him away and then treated him as an equal member of the tribe.
Can someone explain why they may have done that? And more generally what was it about Native American cultures that made them open to adopting outsiders since I know of many other accounts of natives adopting (usually adult) Europeans from the 16th-19th centuries?
Unfortunately, the only information we can get about this event comes from Lehmann's second (and approved) autobiography "Nine Years Among the Indians." I say unfortunate because one, it only gives one side of the story, Lehmann's, who no doubt had a lot of inherent biases against the Native Americans (NAs) and two, was written almost 50 years after the kidnapping period, meaning some of the details and recollections about Lehmann's capture could very well be misremembered.
Taking these into consideration, it's important to note that no Dances with Wolves scenario occurred: he was not treated as an equal in their tribe. Lehmann notes that he was tortured and kept mainly to be a slave for the NAs, particularly the Chief Carnoviste. Lehmann even hints that he was sexual abused:
"We herded the horses and I waited on my master, Carnoviste, the chief; he stole me, so I belonged to him. I would get his horse, bring his food, light his pipe, bathe his feet, paint his skin, tighten the spikes on his arrows, catch lice on his head and body, and attend to what other chores he required, some not decent to put in this book."
Regarding your second question, from my understanding (you'll have to consult a true historian on the topic since I'm average) it was actually very uncommon for NAs to adopt orphaned white children. That idea seems to have been popularized and spread through James Fenimore Cooper's fictional Leatherstocking Tales series, which sees a white child (Natty Bumppo) adopted by NAs.