Let's say you're reading a story about a group of friends who are planning a surprise party for one of their friends. With third person omniscient, the author can take you inside the mind of each character, so you know who's hiding the party from whom, and who's getting cold feet about the whole thing. It's like you're getting a behind-the-scenes tour of the story, and it's a lot of fun.
For example, the author might write something like: "As Sarah was busy making the cake, she couldn't help but think about how much she hated surprise parties, while John was secretly worrying that the whole thing would be a disaster." See how that works? The author is jumping back and forth between characters, giving you a bird's eye view of the whole story.
This technique is especially useful when you have a lot of characters, and you want to show how they're all connected and intersecting. It's like you're looking at a big spider web, and you can see how every thread is linked to every other thread. It's a great way to create a sense of complexity and depth in a story.
