Since the footer is the last visible thing on the site, we can expect that people prepare to leave. We can think of it in this way: "What is the last thing we would like to say to a guest visitor before he/she leaves?" Not all designs can give a satisfactory answer here. If well-designed, this last thing can leave a lasting impression. This is the reason why some designers try to provoke a smile at the end—through a small cheerful illustration or something else—to uplift the mood of the user in a memorable way.