If there was a tick box somewhere that allowed us to mirror our avatars it would be slightly easier to tell people apart, effectively doubling the number of unique images. If we were told how many accounts used the flipped image vs the original, we could deliberately choose the less popular direction to help distinguish ourselves.
It's always mildly irritating to read a conversation between two people with the same avatar or to encounter anybody with the same avatar as me, and flipping seems like a good way to halve that problem.