Cleanliness of body and purity was considered to proceed from due reverence to God. This reference is related to cleaning one's hands before eating and ceremonial washing. It is also given in John when Jesus was in the fields eating wheat without washing his hands, and Jesus provides the example of the cup which is clean on the outside but filthy on the inside.
Also, a clean heart is the result of having God in your life. The Bible tells us that if we are not clean, we will not be able to remain in God's presence.