You're right. An example use case.
Monitor having issues. Google solution. Land on a forum, but to see the full post / solution it requires email registration. I register with a junk yahoo type email address. Complete the long form, solve all the traffic lights, etc. Then get the solution, make a few posts and probably forget about it.
Monitor having problem again after 2 years same forum but it says my very unique username is taken. Now, I vaguely remember creating an account but don't remember what email I used. I try to reset my password but dang, each time it says "If that email was in our db you'll get it". If I get a hint I used yahoo maybe I can resume and hopefully use my old account and some post count than starting a 1 day old account with 0 post.