I'm on a school chromebook wanting to watch people's runs but I can't because twitch is blocked and it doesn't show up. Does anyone know the reason for mandatory VODs?

to prove that your times are real

If you're asking why people submit Twitch VODs instead of another format - people are allowed to submit other formats. I submit my runs as YT videos. It's a matter of preference (although it is required that certain sites be used to prevent things like having videos suddenly become unavailable).