Why does the Pasuk refer to the census that took place when they left Egypt?
Rashi: To stress that the census here, like the census there, 1 should include all those from the age of 20 and upwards.
Seforno: To stress that the census here, like the census there, 2 should be done "l'Veis Avosam l'Mishpechosam".
Bechor Shor: It is because inheritance of the land depends on Yotzei Mitzrayim, according to all opinions.
Alshich: This census was in order to testify about the Kosher lineage of Bnei Yisrael (refer to 26:5:2:1). Do not say that youths below 20 were excluded, for their lineage was tainted. Rather, it is because it is like the first Minyan.
Ha'Emek Davar: Here it does not say how they will be counted. There, it says "b'Mishpar Shemos l'Gulgelosam."
Rashi writes that the Pasuk refers to the census after Yetzi'as Mitzrayim to teach that it from 20 years and above. Our verse explicitly says "mi'Ben Esrim Shanah va'Ma'alah"!
Hadar Zekenim: It is to teach that also here, they counted through half-Shekalim. Meshech Chachmah
Since Elazar is mentioned, surely this was after Aharon died on Rosh Chodesh Av. If the Torah is in order, this is before Sefer Devarim, which begins on Rosh Chodesh Shevat. We give the [half-]Shekalim in Adar, for the coming year's Korbanos. Sefer Devarim begins on Rosh Chodesh Shevat! Perhaps the Torah is out of order, and this was just before Moshe died, on Adar 7. Or, it was not yet enacted to give to the Shekalim in Adar. Or, Hashem commanded to give the Shekalim early this year. (PF)
In our texts, Rashi concludes 'v'Gomer', i.e. he alludes to the continuation of the verse! (PF)