YouTube makes videos available in a bewildering range of formats and
codecs, from the cutting-edge to some old “legacy” formats that are
still in common use. As new codecs become popular, it goes back
and re-encodes existing videos to reduce its own bandwidth costs--this
on top of the transcoding required for every new upload. It has decided
that this never-ending transcoding burden now requires special-purpose
hardware, and has published a paper on the subject.