Giving your creature indestructible can be a lifesaver if you need other stax pieces (I am assuming if you play Boromir, Warden of the Tower , you are playing a stax deck) to stay alive, or if you just want to beat down someone and survive a board wipe
Part of adapting to Play Boosters included making less unusable commons in Draft, which made the fifteenth card less necessary
The art is pretty saucy, too
Its quite common in burn decks to supplement sources of non-combat damage with Hasty creatures that can get under opposing removal spells
At the most basic level, it can be a pseudo-protection spell by switching auras and equipment away from a creature thats about to die and suiting up something else
An effective card against midrange creature decks, Soul-Scar trades places with Monastery Swiftspear as the 1-drop in many red Pioneer decks