When the ash is to be taken outside the tabernacle and outside the camp of Israel, the priest is to change his clothes and put on ordinary garments. This was both practical as well as symbolic. By changing their garments, the priest would not soil their official garments, thus maintaining their symbolic holiness.
4 Puis il quittera ses vêtements et en mettra d'autres, pour porter la cendre hors du camp, dans un lieu pur.