In practice, it's almost identical to what youd do, which is cast to counter their spell, untap, and attack, making the difference nearly negligible with the big upside of having a 3/3 flying creature, which is extremely good
As Silumgar's undead vizier, she strove to ensure her own power and status
When you do, target creature you control can't be blocked this turn
Once you get to the late game and things are going according to plan, having a Dissolve in your hand means there's very little your opponent can ever hope for in order to claw back into the game
Assume you use Diantha on Mega Gardevoir EX, you are left with one Energy, and you can then attach the following turns Energy to it and continue attacking
This immediately doubles the count of collectible items for many cards