How would you substitute certain actors in an existing map without using a mutator?
Lets say you created a new heavy soviet tank and want it to replace the IS2 on all maps it occurs. That could be done with a mutator.
Another example, would be a mutator that changes the player loadout. Lets say you want a mutator that lets all players spawn with MGs.
The ModifyPlayer function would be it then.
Imo mutators are a method to change small aspects while leaving the rests untouched. New actors (when you create them by subclassing) would always need to be placed by hand in new maps (if they are placeable) or called from some other class, which you, of course, would have to modify to do so too, probably resulting in a new gametype.