“A man of your descendants in their generations who has a blemish may not draw near to offer the bread of his G-d!” (Vayikra 21:17)
It is a negative commandment that a blemished priest may not serve in the Temple, as it says “A man of your descendants in their generations who has a blemish may not draw near to offer the bread of his G-d!”. This mean to say that he may not draw near to serve, for all affairs of food are called bread in many places.
Amongst the roots of this commandment are that most of the actions of a person are measured by their viewers according to the importance of those who perform them. It follows that a person who looks important and has a reputation for good deeds, will find favor and good will in all that he does in the eyes of all his beholders. On the other hand, if he has the opposite of the above characteristics, meaning that his form is unsightly, he is deformed in his limbs and his actions are not truthful, his actions will not find favor in the eyes of his beholders. It therefore follows that the messenger upon whom the atonement of each person depends should be a man of beautiful form and appearance whose actions find favor in everyone’s eyes, so that people’s thoughts should follow him. Apart from the above, a man of perfect form is an allusion to the fact that since people think about this person, his soul will be purified and elevated, and it follows that it is unfitting that there should be any deformation in his body, lest it disturb the thoughts of the thinker and he will lose concentration from the object.
This commandment applies when the Temple is standing to the priests. A priest who transgresses this commandment, whether deliberately or inadvertently, causes his service to be invalid.