If nothing else, this lets you make loading spinners with proper alpha-blended antialiased edges (so you don't need "dark background" and "light background" versions).
Indeed, as far as I know, most video formats currently playable in browsers don't support transparency at all.
Do you have examples where APNG wins compression-wise relative to H.264?
JPEG usually wins over PNG compression-wise, but in some use cases it makes sense to use PNG.
The same thing with animation/video.
Suited in terms of desired quality, not necessarily file size. Sometimes you need pixel perfect frames, not 4:2:2 b-frame messiness.