A practice usual in those parts, Genesis 19:2, Genesis 24:32 43:24 1 Thessalonians 13:4,5 1 Timothy 5:10, because they used to travel either bare-footed, or only with sandals to cover and secure the bottom of their feet.
Related Commentaries of Genesis 18:4
Genesis 18:4
4 Let a little water, I pray you, be fetched, and wash your feet, and rest yourselves under the tree: