many, many writing teachers will tell you to avoid third-person omniscient at all costs. They’ll break out the pain sticks. Why is this? Because third person omnisicent is so freaking easy to mess up. And if you crash and burn with an omniscient narrator, you leave an extra-large crater. How can you write an omniscient narrator without being omniscient yourself?
First, it’s easier to be funny with third person omniscient.
Second, it lets you info-dump.
Third, it gives you versatility.
Fourth, you can have a narrator with a personality.
And finally, it lets you tell stories about more than one person.
these are only the bullet points. head over to io9 for the full, much more detailed text. as a commenter suggested, the great Charlie Jane Anders [an accomplished writer herself] should gather all her texts on the craft of fiction and publish them in a book.