To avoid deep binges into youtube. First of all, youtube thumbnails are cancer anyway, so blocking them is one step towards a Lower Information Diet. And the first step to being able to quit any addictions to youtube.
YT pressures content creators into doing all sorts of stupid shit. They are made to think they don't really have a choice unless they want to risk their monetisation.
One memorable particularity was YT prompting the content creators to make thumbnail images with grimacing faces, which IMO is cancerous. yt/user/theCodyReeder talked about it, but I cannot find the video any more, this was about two years ago.
When YouTube (actively or not) incentivizes that style of thumbnails you cant really blame the creators. I've heard something like 20% more viewers based solely on having a more "creative" thumbnail.
It drastically reduces the occurence of me clicking another video.
If I want to see a video I have to read all titles and it gives me a chance to think if I want to stop watching.
An added bonus is that it takes me at least a couple seconds to find a video to watch when I open the front page and I don't end up subconsciously clicking the first interesting thumbnail.
PS: Also let me add that I'm using youtube signed off with no cookies in a browser that doesn't remember history so the front page is usually the same videos that I don't want to watch for a couple days or a week at a time.