Because identifying as part of a group is part of the mechanism that lets humans act collectively. Same kind of thing as the need to belong to some group, organization, or clique. Humans acting collectively are safer, stronger, better fed, and in general reproduce more successfully than humans who do not.
Being able to essentially choose a group to belong with is beneficial - it's a shortcut to most of the benefits of group membership (in ancient history, I mean) without having to be born into the group or be one of the "O.G." members. There are advantages for the rest of the group, too - they can get more people on "their side" and committed to their welfare without having to birth/raise/feed them.
Being able to essentially choose a group to belong with is beneficial - it's a shortcut to most of the benefits of group membership (in ancient history, I mean) without having to be born into the group or be one of the "O.G." members. There are advantages for the rest of the group, too - they can get more people on "their side" and committed to their welfare without having to birth/raise/feed them.