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 httpsHTTPS 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-protocolHTTPS protocol is probably this. Since googleGoogle probably re-uses certain cookies and other information between their sites, it extra important to protect it so one can't break into your gmailGmail for example by using youtubeYouTube as an entry point.