What demos are?
A file type (with the .dem extension) containing recorded events that can be edited and played back in-game. Demos can be edited to change camera angles, speed up, slow down, play music, and other functions. (from Valve wiki)
- You have your every game archived once enabled. Something nice happened to game? You can always get back to it!
- It takes way less size than a video and about no system resources compared to its recording.