Why do swimming shorts have a liner in them?

The following might have a bearing on this question.

I was wearing the shorts you see in the photo at Grimsby Swimming Baths one day and was standing on the side of the pool talking to a friend. He was looking up at me and suddenly he said, “Pat, I can see up your shorts” This was embarrassing as if he could see then so could everyone else. There was no inner slip and he was having a clear view of my male attributes. Thereafter I always wore a red pair of briefs underneath when I wore those shorts.

The provision of an inner liner will prevent the embarrassing situation I found myself in, and contribute to keeping everything in its place.