1)
Why does the Torah need to add that Moshe counted them in Midbar Sinai?
1.
Ramban (on Pasuk 18): To teach us that although he began to count them immediately, 1 he did not manage to complete the census on the same day. 2
2.
Rashbam: To distinguish between this census and the next one - in Pinchas 26:3 - which took place in the Plains of Mo'av.
3.
Oznayim la'Torah: To teach us that the census was completed whilst they were still in Midbar Sinai, before they left on the twentieth of Iyar.
