> The larger picture is that you can use PNG or SVG anywhere you need transparency, which are also better suited for the content that has transparency.

It is pretty common to have anime characters where you remove the background to use as reactions, and while you can use PNG in those cases it makes the images huge. WebP makes it ideal for those cases, this is even the case that I saw this being used in some image boards (specially also including animation, another thing that is popular).

Just because you can't think in a use case there isn't mean that there isn't a use case.