Sforno on Numbers 1:18/ספורנו על במדבר א׳:י״ח

1ויתילדו 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.