Multiple media formats must be used

Because it's possible the video might not play on other browsers.

Different browsers support different video / audio formats AND different Media Container Formats

A WebM container typically packages Vorbis or Opus audio with VP8/VP9 video. This is supported in all modern browsers, though older versions may not work.
An MP4 container often packages AAC or MP3 audio with H.264 video. This is also supported in all modern browsers, as well as Internet Explorer.
The Ogg container tends to use Vorbis audio and Theora video. This is best supported in Firefox and Chrome, but has basically been superseded by the better quality WebM format.