When you want to stay updated on the events of a show but don't actually feel like watching it, so instead you check its tags on a social media website (often Twitter or Tumblr) to read people's comments about it and get an idea for what's happening. It can also apply to a show you actually do want to watch, but can't for whatever reason, and want to know what happens, anyway.
Friend A: "I don't really care enough about Glee to watch it anymore, but I still can't help but wonder what it's like these days."
Friend B: "Why don't you just tag-watch it? There are still plenty of bloggers that care about it, for some reason."
-OR-
Friend C: "My roommate was a dick and invited his girlfriend over during The Walking Dead, so now I'm sitting outside the door, reluctantly tag-watching tonight's episode and trying to ignore the sounds of him porking her in his bunk."