Portability and not having to install the app is a huge advantage. Especially on operating systems where there aren't any decent choices. Take Android for example, the Play Store is full of rubbish and adware-riddled apps, finding a decent app in there is like looking for a needle in a haystack. And whilst FDroid exists, most of the apps there are pretty basic in general, especially wrt this example (video editing).

Putting aside the video editing example for a bit, consider the photo editing web app Photopea, which is an excellent alternative to Adobe Photoshop. Linux is in urgent need of a Photoshop-like editor (and no, GIMP doesn't cut it), but Photopea does a decent enough job for many amateurs and even some pros. For a lot of these folks, Photoshop is one of the last things stopping them from switching to Linux, so apps like Photopea fill that gap. And guess what, Photopea works great on Android too.

Another use case is restricted environments where you can't easily find and install apps, eg immutable distros, or work computers. I use Photopea on my work PC quite regularly for light editing, because MS Paint sucks, and my role doesn't really justify going thru the hassle of getting the approvals to get an editor installed. So like it or not, web apps have their place.