Why do you keep asking the same question? Every time someone gives you a reason you just hand wave it. Berath doesn't need the Watcher, they're just the best person for the job. They know the true nature of the gods, they've proven that they're capable of great things, and they have a personal stake in it since Eothas literally killed them (waiting for you to claim that's an "insignificant" reason).
And it's not an insignificant part of the Watcher's soul, the implication is that you've been heavily weakened without it. Also there's multiple dialogue options that demonstrate a "vengeful" Watcher.
It's an RPG. That's for you to decide, the game gave you plenty of chances to create one for yourself. Berath forcing you, being a goody two-shoes, finding out what Eothas is doing, revenge etc etc.
I have my own complaints about the god awful critical path in POE2, but proper justification for the Watcher has never been one of them. There would be no reason for the gods to hold you in such high esteem otherwise.