Anecdotally there is a sizable portion of customers that want to ask a question without reading much or doing anything or worse having to call or fill out a form, so the prompt and connecting to a person on chat is very effective and quickly gets them to next steps / answers.

It comes down to implementation and the gripe about nagging is fair, but the utility of the thing is still valid and like another commenter mentioned it's a great thing that's happened to customer service.