#1 - June 14, 2013, 8:04 a.m.
Let me use the spell Corruption for example. The text states that it must target an enemy minion. If there are no enemy minions to target, can you still play it? Obviously it doesn't DO anything, but can you still pay one mana and play the card?
The closest example I can think of is with minions. Take Crazed Alchemist; if you play him and there's no other minions to target, his Battlecry just takes no effect, but you can still play him. Similarly, can you "play" the spell with no target and basically just have it do nothing?
The number of situations this would matter in are of course limited, but it would matter for example with proccing Mana Addict's +3 attack bonus, and it would proc Illidan's Flame of Azzinoth.