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by thaumasiotes 1014 days ago
A .webm file is an .mkv file. You can rename to .mkv without any ramifications.

My guess would be that yt-dlp is giving you whatever file format it finds, with no editorializing. The default is just to go with "best quality" available. So "Is there a reason for this?" is likely a better question for whoever maintains the site(s) you're downloading from.