In Thor: Ragnorak, Odin states that Hela draws her power from Asgard, so does Thor (if I remember correctly).
On Earth her costume has multiple tears, signifying her weakened state (maybe it's result from million years imprisoned far away from Asgard). When she arrives on Asgard her outfit is now whole and takes on her traditional green color scheme, showing that she has regained her strength.
That's why unleashing Sultur was an option, so he would destroy Asgard, in order to weaken Hela, and therefore end her.
So, if Thor also draws power from Asgard, how is he still powerful and kicking bad guys in Infinity War after Asgard was completely destroyed?