The main purpose of multi-tasking is to improve one's overall efficiency.
But multi-tasking isn't necessarily the only way to achieve that goal.
In the context of watching anime, for example, I can as well speed up the playback, so that I can finish an episode within less time than usual.
Considering I don't have multiple cores in my brain like my processors do, it is impossible to eliminate the possibility of missing important anime scenes if I am distracted by other stuff.
Should that kind of accident happen, I will have no choice but to move backward and repeat those missed scenes.
And this story isn't over yet, that's said, unlike watching anime which I can always fully recover the unfavorable results of distraction, some jobs can't be taken that easily.
If I make a major mistake in my other task because of the anime, it will cost me even more.
So?
If you ask me what do I do while watching anime, I will give you my definite answer: nothing.