A recent Youtube Web update has added a canvas whenever the seek bar is visible, an HTML5 canvas pops up. This was not asked for and not needed. If you disable canvases for privacy, this will cause a horrific red bad to cover half the screen until you hide the seekbar. Canvases can be used for fingerprinting, which I’m sure Google is doing here.
It’s probably more of a scale thing, going a conversation server side need CPU time, if it can be done prior to upload then server time is reduced. I think a lot of websites do client side processing so they can do more requests per server instance.