As Andrew said, it's a way people can interact (and subscribe) on YouTube. I thought a cute little dinosaur play button subscribe call to action would be a fun bit of branding. It adds a little bit of personality, which in turn, could make it more likely someone gets curious, notices it, and clicks on it, potentially winning you another subscriber. Most of the time I find them kinda "meh" they're a business brand for most, so I pay no attention to them, BUT if you can do something clever of funny with yours, like I think I did with mine, it can be a little bit of extra personality.
I'm heavily considering hard watermarks, where you burn your watermark, typically into the bottom left corner to keep it out of the way, but also to "protect" your videos. In the art world, and everywhere for that matter, people WILL download and use your content, weather you allow it or not. Now, even with a watermark burned into the video, people can still edit the video to get rid of the watermark with a blur, BUT, if you're more keen to accept that sometimes people will just take your footage without permission... you might as well get some credit, and have your watermark in there so whenever they use your footage in other videos (especially in shorts or compilations) people will at least have a better idea of where that footage came from. This is why you see a lot of media adding their watermarks onto their footage burned in so they can get a little bit of extra credit when their content inevitably goes beyond what they can control. A lot more important for artists, and those who have sharable content, but if you can make it small and out of the way, I think most can get by with havving a small watermark in the side.
Have your watermark transparent at somewhere around 3%- 5% opacity, that usually keeps out of the way, but is still there, but of course, experiment to what works for you.
I've been dealing with watermarks for over 15 years, it's why McDonalds puts a little (c) or a little TM over everything, so you know it's there.