You don't subscribe to Youtube. You just register and log in, as you would with any site.
If you don't register and log in, the content you get will be randomized for the general population. Your feed will be filled with whatever happens to be the most popular, which in my experience is mostly junk. Registering an account allows them to tune the content according to your preferences and viewing habits. Although this has downsides (echo chamber effect), I'd rather have that than a feed based on popularity. Registering an account also allows a lot of functions you don't have otherwise (e.g., saving videos to watch later, watching a long video in segments rather than having to watch it all the way through, having your feed full of stuff from content creators you like, participate in discussions, etc.).
If you just dip into YT now and then for a specific purpose (e.g., "search cat videos"), that's different. Then you don't need to register, any more than you'd need to register at this site if all you wanted to do was check to see what people were saying now and then. I watch a lot of YT content, though, so I'm happy to register.
p.s. fwiw, the email address appearing isn't my actual email address. It's some odd hybrid. Not sure why that is. Maybe it's a goofy version of it that I registered with. I can't remember.