I try to imagine all the dirt, dust, and grime which must have covered the feet of men and women in the olden days when sandals were the main footwear. It's no wonder that foot washing is mentioned in the Bible; it even becomes a teaching opportunity for Jesus.
We are usually quite happy when we can "stand on our own two feet." But the main message of the foot-washing ceremony is that we put our own best feet forward when we serve others, when we take that downward humble path rather than the common path of pursuing power or success. It also reinforces the recognition that our feet connect us with what is sacred.
This is illustrated by a Sufi story from An Emerald Earth by Felicia Norton and Charles Smith:
"The trickster Nasrudin is found sitting in a church with his feet upon the altar. The local priest comes in and shouts: 'Nasrudin, never put your feet on something sacred!' Perplexed, Nasrudin looks around: 'Oh, forgive me! But where can I put them that isn't sacred?' "