Different media players play different file formats.
Some formats work better with assistive devices.
Some file formats are easier to edit.
WebVTT: The Web Video Text Tracks Format http://dev.w3.org/html5/webvtt/
What Caption Format?
From Captioning & Web Video by Mark Gamble SC AT EXPO 3/26/2014
http://scvrd.net/downloads/pdfs/Captioning2014.pdf
(Slides 41-49)
MPEG Streamclip http://www.squared5.com/
A "powerful free video converter, player, editor for Mac and Windows... You can use [it] to open and play most movie formats ... edit them with Cut, Copy, Paste, and Trim; set In/Out points..."
Miro Video Converter (Windows, Mac, Linux) http://www.mirovideoconverter.com/
Convert to MP4, WebM (vp8), Ogg... Android, iPad...
VLC http://www.videolan.org/vlc/ This media player also does media conversion