Biologists have been pondering why homosexuality is so prevalent in the animal kingdom and their explanation for this is a positive evolutionary pressure that benefits the species.

So naturally:

The “gay uncle hypothesis” posits that people who themselves do not have children may nonetheless increase the prevalence of their family’s genes in future generations by providing resources (e.g., food, supervision, defense, shelter) to the offspring of their closest relatives.

Note that it’s only a hypothesis since the research hasn’t been conclusive.

Source: https://en.wikipedia.org/wiki/Biology_and_sexual_orientation#General