Adding the ability for Discord bots to 'go live' in a server via a video stream would allow for things like direct streaming of online media, better streaming from other services (like Twitch or YouTube gaming), and more. I'm sure there's plenty of ways people can implement and use the feature, too.
EDIT: Since Discord are potentially allowing party games via voice channels, I'd like to add another thought - Custom games available via bots, using video streaming. Games from bots have existed, but they're usually lackluster since the only way to interact with them is through text channels. While this would demand more performance out of bot hosting servers, it could potentially make a custom version of what Discord is adding right now, which opens up a lot more ideas.
Also, to address bandwidth issues - You can either A. put a cap on bot streaming (i.e. X amount of streams at once, with a max res of Y.) or B. literally just let the bot stream to users directly.