Here's my interpretation of how the story is changed by not covering that segment of the rabbits' journey in detail.
Off-stage character development enables the plot to proceed more swiftly.
At the beginning of the story, when the band of 11 rabbits start their journey from Sandleford, they're essentially a bunch of misfits, not having much in common, many of them not even knowing each other well. By the end of the story, when against all odds they are able to triumph against the mighty Woundwort and the efficient machine that is the Efrafan Owsla, they must be able to work together perfectly, trusting and respecting each other, knowing and relying on the unique abilities of each one of their number.
How do they change from one state to the other? In reality, this would be a continuous process, as the rabbits slowly learn about each other and their abilities. But it makes for more efficient storytelling to not show every bit of the process in detail. By streamlining and skipping over some parts of their development, the story is able to refer to some of the incidents that caused them all to respect each other and hold together firmly, without needing to describe all those incidents in detail. A vaguely described incident creates a point of reference that can be referred back to more freely in order to justify some knowledge or relation between the rabbits, without the author needing to rewrite the incident to make sure it really does justify that.
One might call this lazy storytelling, and indeed it would be if all character-building incidents were skipped over and then referred back to. But this is not the case. We see several of the rabbits' ordeals described in detail which help to build up the characters and relations between them:
- In the woods in chapter 5, Hazel takes the risk of breaking cover first, and Dandelion, the first to join him, compares him to El-ahrairah. This is an early sign of Hazel's leadership and also Dandelion's speed.
- At the river, Bigwig returns to warn Hazel about the dog, Hazel refuses to cross without Pipkin, and Blackberry figures out how to use the wood. This establishes Bigwig's loyalty, Hazel's responsibility, Pipkin's devotion to Hazel, and Blackberry's cleverness.
- One of their biggest tribulations was their time in Cowslip's warren. Here they were almost divided, but events proved that they could rely on each other and nobody else. Every single one of the Sandleford rabbits came to help Bigwig, and none of the warren rabbits.
The development of their group after these events is described clearly in the third paragraph of Chapter 18:
Since leaving the warren of the snares they had become warier, shrewder, a tenacious band who understood each other and worked together. There was no more quarreling. The truth about the warren had been a grim shock. They had come closer together, relying on and valuing each other's capacities. They knew now that it was on these and on nothing else that their lives depended, and they were not going to waste anything they possessed between them. In spite of Hazel's efforts beside the snare, there was not one of them who had not turned sick at heart to think that Bigwig was dead and wondered, like Blackberry, what would become of them now. Without Hazel, without Blackberry, Buckthorn and Pipkin -- Bigwig would have died. Without himself he would have died, for which else, of them all, would not have stopped running after such punishment? There was no more questioning of Bigwig's strength, Fiver's insight, Blackberry's wits or Hazel's authority.
The incident with the rats in the barn serves as one more event to illustrate these changes. The sentences immediately following the above quote are:
When the rats came, Buckthorn and Silver had obeyed Bigwig and stood their ground. The rest had followed Hazel when he roused them and, without explanation, told them to go quickly outside the barn. Later, Hazel had said that there was nothing for it but to cross the open pasture and under Silver's direction they had crossed it, with Dandelion running ahead to reconnoiter. When Fiver said the iron tree was harmless they believed him. [...] Yet [Bigwig] himself had become less overbearing. The snare had left him weak and overwrought. It was he who had given the alarm in the barn, for he could not sleep and at the sound of scratching had started up at once. He would not let
Silver and Buckthorn fight alone, but he had felt obliged to leave the worst of it to them. For the first time in his life, Bigwig had found himself driven to moderation and prudence.
The incident doesn't need to be described in detail in order to serve the purpose of illustrating how the rabbits now rely on each other, even Bigwig relying on others to do the fighting when necessary. Not providing all details about some events allows them to be used as evidence for any piece of character development not specifically illustrated by other events described in more detail - such as Silver and Buckthorn's value as fighters.
Not describing it in detail also helps to build contrast between the before and after states, between the rabbits who bickered about what to do or where to go and the rabbits who rely on each other's abilities unquestioningly. If all their character-building moments were covered in detail, there would be a lot more chapters between the before and after, and the change would be as hard for readers to observe as perhaps it was for the characters themselves.