Picking the space character, apart from being I think an established convention already (the CHAR(n) type in databases), definitely has the advantage you can just "blit" the filename onto the video buffer in text mode when doing a DIR command. You don't have to bother with checking its length or anything, just copy 11 bytes and insert a space (not dot in the original DIR listing format) after the 8th one. If the file doesn't actually have an extension, this just prints the right number of spaces so the other columnms are aligned.

EDIT: see https://en.wikipedia.org/wiki/Dir_(command)#/media/File:Spar... for an example.