The land they have just conquered is great for livestock… Reuben and Gad are vast herdsmen tribes and they want it, not to go into the promised land over the jordan.
Moses does a deal where they can have it if they join the fight for Canaan, but if not they will be given land within Canaan. Either way, their lot is with the Israelites, fighting for Canaan.
He compared it to the weakness of the generation who lost their will to go to the promised land after the spies report. Its easy to see a lesson about settling for instant gratification and not pursuing God’s plan.
As you will see from my reading of Joshua, the promised land project for me, and for many I’m sure, keeps being tainted with sentiment for the occupants of the land… its not empty. So I have a mixed reaction to the chapter.
But certainly you have this sense that God’s people are bound up with each other, the mission of one group is the mission of all, and they must not be distracted by the dazzling opportunities along the way to obeying God’s will.