Originally Posted by Renegade Rook
Every time kat gets a kill her cds are reduced, one kill refreshes her q w and e, two kills refresh her R. Trist's jump refreshes if she gets a kill, Kha'zix's jump only refreshes on kill if he has that skill evolved.
He wasn't asking about the reset, He's asking why they get a reset?
OP you missed darius and he isn't an assassin. I think it's more of a niche thing, all 4 of the ones you listed have their gap closer/escape refreshed to keep them safe/viable.
For the darius reset case I think it's more of his only way to do some nice damage in team fights, otherwise he just does nothing