A.T. Robertson Commentary Mark 1:44

"and saith unto him, See thou say nothing to any man: but go show thyself to the priest, and offer for thy cleansing the things which Moses commanded, for a testimony unto them." — Mark 1:44 (ASV)

For a testimony unto them (εις μαρτυριον αυτοις). Without the formal testimony of the priests the people would not receive the leper as officially clean.