Who exactly was included in the census?
Seforno: It incorporated conscripts between the ages of twenty and sixty, as men over the age of sixty did not go to war. 1
Seforno: See Bava Basra, 121b.
If it was necessary to count the tribes as is the way of kings who order a census, why did Hashem instruct Moshe and Aharon to find out the sum total of the entire nation?
Ramban #1: So that the people should be aware of Hashem's great kindness, inasmuch as, when they arrived in Egypt they numbered seventy, and now, they were numerous like the sand of the sea, so many from the age of twenty and upwards. 1
Ramban: It was in order that every person would be able to pass before Moshe and Aharon, who would gaze upon them with an Ayin Tovah and bestow upon them a Berachah. 2 And the half-Shekel would atone for their sins.
Ramban #3: Because they were poised to enter Eretz Yisrael, to begin the conquest of Cana'an, 3 and as is the way of an army going into battle, 4 Moshe and the princes needed to count the soldiers, to know the sun total of fighting men and how many there were in each tribe. 5
Ramban #4: Because they were also about to distribute the land, and it was necessary to know the numbers, in order to know into how many portions they would need to divide the land.
Not even counting the women and children.
Ramban: 'Hashem Elokei Avoseichem Yosef aleichem kachem Elef Pe'amim vi'Yevarech Eschem ka'asher Diber lachem!'
Ramban: Which is what the Torah is referring to when it wrote (in Pasuk 3) "Kol Yotzei Tzava be'Yisrael".
Ramban: So that when they arrived at the Plains of Mo'av, they would be able to present them with their battle orders, since the Torah does not rely on miracles.