Or HaChaim on Genesis 19:1/אור החיים על בראשית י״ט:א׳

1 ויבאו…בערב. They arrived in the evening. The reason for this whole story of the angels being hosted by Lot was to afford Lot the opportunity to justify his being saved. Although Bereshit Rabbah 50,11 attributes Lot's escape to Abraham's merit, we still need to find some redeeming feature in Lot's conduct to understand the efforts made by G'd to save him. Had the angels arrived at Sodom in daylight Lot would have lacked the moral courage to offer them hospitality even by devious means.

2וירא לוט ויקם לקראתם. Lot saw and rose to meet them. This means that Lot recognised that these people were of the type familiar to him as ministering angels of the Lord when he still travelled with Abraham. Because Lot realised this he was prepared to take risks for their sake against the inhabitants of his town. He told them to wash their feet, a custom he had learned from the way Abraham conducted himself with his guests. The Torah reports the details to show the difference between Abraham and Lot. Abraham commenced with his guests washing their feet, Lot suggested they sleep first. Lot did not mind if people who were idolators entered his home without first removing the traces of idolatry, something that Abraham was very particular about.