The Chidushei Harim zy’a asks if the tribes of Gad and Reuven originally wanted to join the war, why didn’t they say so in the first place? Why did they only tell Moshe that they planned to do this after he chastised them?
He answers that they didn’t want to interrupt Moshe Rabenu because they knew that they could benefit greatly from his mussar. They didn’t want to lose their opportunity to hear words of mussar from Moshe Rabenu.