Quote from FailingAtFailingIn my view, the point of harassing is to slow your opponent down, but if you send your whole army in, they could get killed, especially if they are hiding something like a magikill. If I play it safe, I run less risk of losing parts of my army.
However u will destroy maybe all of their army, and all of their miners if they don't have miner wall or don't garrison. And at that point, u should have giants coming in.