It's a way for writers to show the villain as a legible threat and quickly raise the stakes up. Like in Annihilation, depowering Galactus was a quick way to show that Annihilus isn't messing around. He's a dangerous threat.
My favorite Batman book is Arkham Asylum, so Morrison, even though he wrote some over the top stories with Batman in it. But Dini is up there, because he can take the Batman universe and make it easily digestible while keeping it true to what it is. Frank Miller get's a nod, for Dark Knight Returns alone.