By discarding it, you can have a Thunder Monster gain 500 Attack
Certain exceptionally powerful Pokemon called V, VSTAR, and VMAX are worth two, two, and three respectively
While not truly comparable to Frilled MysticFrilled Mystic, since the familiar can only counter creatures, if you're holding up four mana and nothing comes around that's worth countering, you can still choose to surveil 3 or remove a pesky enchantment
During regular combat if the attacked monster changed somehow (it went into hiding, changed control, moved, fused, etc.) a replay occurs (meaning the attacking player must choose an attack target again after that first attack was declared)
Lets go into why this is the case
How Hit Kings Compares