Your allies are automatically added to a shared leaderboard so you can comptete directly against each other.

On completiing a solution you can also look at solutions provided by your allies which ycan lead to further discussions.

Can't think of any other uses beyond those two.

I think it's just for fun, like "codewars", making "groups", "allies" and "wars"... I don't think there is any practical point in the allies system..