In some cases (can't speak about this specific case) it's a matter of being recognizable. Many platforms (YouTube, twitch, ..) have limited capabilities for the creator to be recognisable. With such a branding at the second video you know "it's the anime guy"

creator here; That was my desktop background at the time, recorded the video with obs at 720p because I wanted to get anything watchable on youtube (480p suxx if I wanted to include a timer, also it would look way worse).

BTW, she's not my `waifu`, I just like her as a character.

Football fans wear football team shirts to advertise their affiliation. I expect this is the equivalent for fans of Japanese cartoons.

Just a playful way to mess with people. At work, I see a guy that wears a D-Va shirt every other day. He is a big guy, and the whole D-Va thing is printed full-screen on his shirt. It makes quite a funny sight.

4chan people are notorious for pulling that kind of shit. I wouldn't be surprised if the person that makes that video is from /g/.

If you don't find it funny, then you don't find it funny, like how you don't get some movies or memes that people like. I do.

To show off their waifu of course!

Visual appeal. The anime waifu is more eye catching than a plain background.

I just want to know what window manager that it.

XFCE4 w/ Mofit (misspelled in the package) theme. I'll move back to a tiling WM one day..

Probably fvwm.