The torrent protocol downloads chunks in random order. You can't play until a file is complete since there are chunks missing.

If it's a multifile torrent, then it's possible to, say, prioritise certain files over others, and when these files are complete, you may play them.

Download rates are not great unless there's a seedbox among your uploading peers.

Typically seed/leech ratios aren't great either, particularly for older stuff.