פירוש על במדבר 1:18

ויתילדו על משפחותם THEY DECLARED THEIR PEDIGREES AFTER THEIR FAMILIES — They brought the records of their pedigree and witnesses to confirm the prevalent presumption regarding their parentage, so that each might establish his pedigree with regard to the particular tribe (cf. Yalkut Shimoni on Torah 684).

AND THEY ASSEMBLED ALL THE CONGREGATION TOGETHER ON THE FIRST DAY OF THE SECOND MONTH. Scripture mentions [the date although it is already stated in Verse 1 above, to indicate] the zeal of our teacher Moses in [fulfilling] the command of G-d for on the very day of the communication [that he was to take a census of the people] he took the princes of the tribes and assembled all the congregation and began to count them. The count, however, was not completed in one day, and therefore He states again, so did he number them in the wilderness of Sinai,⁵⁷Verse 19. in order to inform [us] that the counting was all done in that place, but was not all [finished] on that day.

ויתילדו על משפחותם, seeing the purpose of the count was to know who would have to join the ranks of those going to war, it was important to have genealogically pure men, not any who were the product of forbidden unions. Knowledge that they were the children of unblemished parentage would protect them when exposed to the dangers present in any war and battle. We find a similar thought expressed in Kidushin 76 that “the credentials of people recorded in the king’s register need not be investigated any further.” On folio 70 of the Talmud there we also find the statement that the Shechinah does not rest protectively except over genetically “pure” families.