פירוש על במדבר 31:13

ויצאו משה ואלעזר הכהן AND MOSES AND ELEAZAR THE PRIEST WENT FORTH, because they saw the young men of Israel going out to snatch the spoil (Sifrei Bamidbar 157:5).

אל מחוץ למחנה, so that the returning soldiers would not immediately enter the camp, seeing they had become contaminated through contact with dead Midianites.

They went out to show honor to the army, as is accustomed even today—to give honor to those returning from a war. However, a king does not normally give honor to his subjects in person, and that is why the Sages said: This informs you of Moshe’s humility.

ויצאו משה ואלעזר הכהן, “Moses and the High Priest Elazar went out to welcome back the soldiers (before they reached the camp);”they could not admit them to the camp because having killed human beings they had all become ritually impure and required seven days of purification.