In the wild, and in sanctuary, primates need a mother figure. Depending on the primate, in the wild they are dependent on their mother anywhere from 1 to 7 years. Great apes tend to be on the higher end naturally. Chimps don't actually get weaned off of breast milk until they are 5 years old in the wild. Needless to say, the mother is the most important figure for a healthy primate society.

In a sanctuary setting, it's not always possible to have natural adoptive mothers. The Vervet Monkey Foundation has a system where wild monkeys adopt rescued babies, but they are the only ones who do it consistently as far as I know. Since most great apes don't have the numbers to be able to mass adopt other children, it's up to humans to teach the young apes how to be apes.

This is why you'll noticed a lot of these sanctuaries employ females to help with taking care of the apes. They do recognize the difference between males and females. Men are typically for socializing, while females are typically for nurturing. That's not to say men can't be in nurturing roles as well, but ideally humans wouldn't need to be doing any of this in the first place.