Our narrator has it made. He can roam around as he pleases, entering characters' minds when he wants to or zooming out for a bigger picture. Whoever he is, he is also a bit of a tease. The narrator can read characters' thoughts (like Macomber's fears or Wilson's disgust), but he will not give us everything. Nah, he would rather watch us squirm.
We do not, for example, know whether Margot shot Macomber on purpose, or by accident while trying to defend him. That's a pretty big mystery to end on, right? Still, we're thankful for moments when we do get some access to the characters' thoughts, because those moments give us insight we might otherwise lack. For example, knowing that Macomber experienced anxiety all night long before the lion hunt tells us exactly why he ran when he faced the lion.
We know that Macomber looks the part. He is handsome, and wearing that crisp, new safari outfit. But appearances can be deceiving, so when we are given access to his thoughts, we know that he's really quaking in his boots, no matter how he might seem outwardly.
The omniscient narrator also helps to let us know what the animals are thinking – for example, the lion, whose experience we are able to access (can't say we've ever been able to do that before). This narrator moves into the mind of the lion (much as it moves into Macomber's thoughts), and we get the inside scoop on what the lion sees, smells, and feels. The lion may be prey, but it has its own experience; it is not just an object, but also a living, breathing being that experiences pain and fear. Through this point of view, we know not only what the lion felt upon being shot, but we also know that Macomber had no idea of these experiences – that he couldn't imagine the lion as anything other than something to fear and to kill.
In changing points of view throughout the story, Hemingway contributes to the complex and contrasting emotions of the characters. Without it, we would never know that Macomber was sure his tormenting wife would never leave him, or that Wilson considered the possibility that Macomber would shoot him in the head. Now, if only we could get into Margot's head…