Because most teams didn't write their user-auth code, it's either a framework or another team. But the PO want "Sign in with Twitter" so you hack this together.

It's so they can feed the data into the sales team in case their algo thinks you are a viable target to sell things to.

They know people don't always update their profiles and use throwaway accounts. They nudge you to provide actual data.

"Nudge"? You sound like a project manager

Forcing to create a new password and field out additional fields isn't a nudge