Every class has removal in some sort or form. Tinkmaster Overspark is neutral card available to everyone, there are also class-specific removals like mage Polymorph, rogue Assassinate, shaman Hex etc.
Before you play your big trump card creature, you usually want to make sure that opponent wouldn't have a removal on his next turn.
Let's say our opponent is mage. Next step requires your skill and knowledge: how many removals does this mage have in his deck? I guess this mage runs Tinkmaster Overspark and Polymorphx2. I know he already used one of his Polymorphs. He have 15 cards left in his deck right now, next turn he will have 14. Let's calculate this - 2 threats, 14 cards left in deck. Pretty high percentage!
But things can get even worse: our opponent may be able to draw more cards his next turn. For example, he may have Arcane Intellectx2, resulting in 2 threats, 10 cards left in deck during a single turn! A devastating ~90% to have his removal card. This is not the odds you generally want to bet against.