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.
That's due to content creators, not 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.