The “Maahir’s Gambit” scenario.

Invulnerability (also known as immunity) is always absolute and protects creatures possessing it from all magic completely, as in the case of creatures with the "Immune to Magic" ability (with the exception of the "Magic Immunity" ability), or selectively from a specific element (as an example, the "Tomb" creature). Therefore, the Staff of Sar-Iss cannot be used to remove immunity and harm such units with magic, but resistance can be removed! It is completely removed, allowing you to effectively target troops that are protected by strong magical defenses (mages, partially Sylvan creatures, dwarves with the "Dwarven Luck" ability). In other cases, it is probably more beneficial to give the hero some kind of sword...