Summary: Take their best card, take something that blanks their plays for a turn or two, or take their one out to your best card
Especially if your commander tax is quite high, or theyre just expensive to cast to begin with, making sure your spell actually resolves can be quite important
Monstrosity is an activated ability, which means you can use it at any time you could cast an instant
This is powerful but its also fragile, expensive and depends on a board of mana rocks to do anything
After all, expecting your opponent to concede while you also can't do anything is not a win condition and not the optimal way to play stax pieces
But ask the typical Magic player how to combine both of those things into a competitive deck, and youll probably not get much more than a shrug or head scratch