Then the LORD said to Moses,
The priesthood being confined to one particular family, and entailed upon all the male issue of that family throughout their generations, it was very likely that some or other in after-ages that were born to the priesthood would have natural blemishes and deformities: the honour of the priesthood would not secure them from any of those calamities which are common to men.
Commenting on Leviticus 21:16-24
And the Lord spake unto Moses,.... After he had spoken to him of the holiness of the priests, that they should not defile themselves, neither with the dead nor with impure marriages, he proceeded to add some things concerning blemishes in their bodies, which rendered them unfit for the service: saying; as follows.
Whosoever he be . . . hath any blemish, let him not approach to offer the bread of his God--As visible things exert a strong influence on the minds of men, any physical infirmity or malformation of body in the ministers of religion, which disturbs the associations or excites ridicule, tends to detract from the weight and authority of the sacred office.