Or HaChaim on Numbers 31:49/אור החיים על במדבר ל״א:מ״ט

1-50. ויאמרו אל משה…ונקרב, They said to Moses:…"we have brought the Lord's offering, etc." The meaning of the soldiers saying that after counting who had returned from the battle it became clear that they had not suffered a single casualty was designed to convince Moses that not one of these soldiers who had volunteered had previously been guilty of any sin even remotely connected to the parts of the body on which these pieces of jewelry were worn , and which they had offered as atonement offerings for themselves. The expression ונקרב alluded to הרהורי עברה, fantasies about committing a sin. The purpose of the sacrifice was to atone for any such fantasies one had entertained (compare Shabbat 64). I have elaborated on this theme in my booklet called חפץ השם (on a variety of Talmudical subjects)