Why did the disciples need only their feet washed after they had been cleansed at the central bath?
Because they had already been fully cleansed, not by water but by the blood of the Lamb, they needed nothing more except to wash their feet. As they walked home through dusty streets, their feet became dirty, so only that part required washing afterward.
Les explains that Jesus' words in verse 10 describe one who has been cleansed, not by water, but by the blood of the Lamb. Such a person needs nothing more except to wash his feet. The lesson is that although they were cleansed at the central bath, as they walked home through those dusty streets, their feet would become dirty and consequently needed washing. So the central bath marked a complete cleansing, while the foot washing addressed the dirt picked up afterward along the way.