That's kinda a Sophie's choice :) The first two answers are basically one - If you keep the chain safe, then you must produce blocks reliably, especially when you're among the top witnesses. Anyway, it's the trusted people who keep Hive safe to me.
@tazi as a witness might have something to say here ;)
I agree with put the first two answers into one.
IMHO this is the main goal for every witness and doesn't matter if he is in top 20 or not...
The other tasks depends on concrete person, his skills, hobbies etc.