There are so many useful use-cases. Like... imagine you want to try out a html snippet, without creating an html-file. "Clearing" the default new-tab-page takes some seconds before you can insert your own stuff via the dev tools.

Remote work environment involving screen sharing during calls/meetings. As a Firefox user I don't need an extension for this, but I set the new tab page to blank. I have a separate work profile on my machine, but still, the default page of most frequent / most recent / most whatever pages is too unpredictable for me (i.e. I am not 100% sure how it works and don't want to devote time to understand it completely). I have rarely used it anyway, CTRL+T then typing a letter or two is way faster than trying to spot the relevant box and click it (separate URL / search bars help here as well). A nice image is cool and all, but it may be too distracting, and then there's the problem of unpredictability again: it comes from a 3rd party (extension developer) using another 3rd party (Bing in this case) to fetch the image. Bing being known for its excellence in the less worksafe areas and all... I won't take my chances :)

Lack of visual clutter