Mormon always has two important things to cover in his story of the Nephites. One is the basic history, but the other is the lesson to be learned from Nephite history. In these three verses, Mormon simply deals with the first of those purposes. He notes that Moronihah had been somewhat successful, but hits a stalemate. He has to stop and consolidate his gains.
These three verses cover the history, but Mormon will set up his lesson to be learned by including stories of this war.