You need an account to store favorites, and to follow channels via subscription. That's why I bother logging in. Also, it lets me keep that stupid auto-play feature disabled. I don't know why that defaults to enabled. I'm watching some video, maybe just listening to it from another tab, and the assumption is that I want to auto-play some randomly-similar video after that. No I don't, why assume I do? Being logged in keeps that thing off like it should be.