Under 5E lore, doppelgangers are changelings twisted by the daelkyr Dyrrn the Corruptor. A general belief is that this change is actually primarily psychological. Doppelgangers have a fundamentally alien outlook. They are predators who so paranoia and chaos when not working for a specific Cult of...
Beyond this, personas can have roles within the tribe or community that go beyond skills. The same changeling fighter could have a persona that’s a martial champion of the tribe, a hero who defends them from their enemies. Like Max and Garrow, it’s not a question of when it’s useful...