Video is the heaviest thing most of us ever attach: screen recordings, camera clips, whole talks. It was born in wherever your camera, screen recorder or download came from, and its real weak spot is nothing inherently; every container brings its own quirks.
WEBM is video the way browsers like it: open, royalty-free and built to stream. A Matroska-based container restricted to VP8/VP9/AV1 video with Vorbis or Opus audio. That makes it the right destination when what you need is embedding on the web without licensing worries.