Imagine this story told from the point of view of Hazel or one of the other rabbits. That bunny narrator would use lots of bunny words—hrairoo, Owsla, hraka—and never need to define what those words mean. Or imagine this story told from the point of view of one of the humans, which would result in a much shorter book: "Those bunnies are fighting. Oh well, who wants rabbit stew? Someone grab my Winchester."
Because the narrator here is third person omniscient, the narrator can stop the action at any time and tell us everything that we need to know in order for the story to make sense. We're not restricted to a rabbit's-eye view of the world, but can move back and forth from rabbit to human perspectives when the occasion calls for it. So the narrator can tell us what it's like for a rabbit to walk uphill in comparison to a human, and come to this conclusion:
The rabbits' anxieties and strain in climbing the down were different, therefore, from those which you, reader, will experience if you go there. (18.17)
The narrator doesn't make a lot of direct references to "you, reader"; but the omniscient narrator can do a lot of comparing between how rabbits feel about things (scared) vs. how humans feel about things (hungry). And that helps us readers (humans) understand what these rabbits are going through. That's why the narrator can get away with putting footnotes explaining rabbit language (or "Lapine"). So the third person omniscient narrator gives us readers all the info we need to know for the story to make sense, without straying too far from the rabbit world and its rabbit problems.
But here's one more reason why third person omniscient narrator helps us: the narrator doesn't restrict us to Hazel's point of view (like a limited omniscient narrator would). This narrator can happily skip over and into… General Woundwort's head (shudder). That's not a fun place to be, but it does help tell us more about this character. After all, Woundwort's not going to have a long conversation over coffee with Hazel about his childhood, so the only way we can get that is for the narrator to just up and tell us.
And so we get a long section in Chapter 34 about Woundwort's childhood (messy), which helps us see both Woundwort's trauma and his good qualities. The omniscient narrator can tell us how brave and powerful he is: Woundwort "was a singular rabbit" (34.3) and "In a month he was big and strong and had become savage" (34.4).
All of which is important to learn because (a) this info makes the story tenser, since Hazel and Bigwig are facing a serious opponent; and (b) this info makes Hazel and Bigwig all the more heroic when they out-smart and out-fight Woundwort. And (c) it tells us that Woundwort isn't a complete monster, but is mostly motivated by his fear of humans, who killed all his family. So at least we know why the guy's such a big fat jerk.