Each one reduces the cost of spells you cast that share a color with it, and they have passive abilities that feed into their own activated abilities
With good planning, you could keep Mew alive despite its low health, and within a couple turns you'd be able to completely wipe out your opponents bench
In a deck without convoke effects, that usually means attacking
This list doesnt include cards that make temporary copies that leave play at the end of the turn like Kiki-Jiki, Mirror Breaker or Flameshadow Conjuring
Any of those will negate Mirror Force upon activation, but the third option has no cost
Of course, the same potential for an OTK still exists with Toon Cannon Soldier as it does with the original one it's banned in the OCG