What’s the use of HTTPS on YouTube? Is that just for the passwords and username not to be grabbed or can it also prevent or hide what you watch on YouTube? From hackers?
If someone would grab the session cookie, that is the cookie that identifies your session after you've logged on, they could gain access to a site as you without needing to break your password. The HTTPS protocol helps preventing this because it makes it more difficult to see the information passed between your computer and the site, thus an attacker can't see the cookie.
So the purpose of the HTTPS protocol is probably this. Since Google probably re-uses certain cookies and other information between their sites, it extra important to protect it so one can't break into your Gmail for example by using YouTube as an entry point.