'A practice puzzling to biologists because it combines apparent futility with near-universal popularity.'
Okay, what do you think this refers to? It's related to human sexual behavior......
That's the fun of being a biologist, you get to ask the questions that other people try to ignore.
Great article here from a few years back that is a bit of an eye-opener if you haven't thought of these things before.
If you'd like to learn more about the 'weird, wild world of animal sex' then Dr Tatiana's 'Sex advice to all creation' is a fun book. Written as a series of sex advice columns for various animals the book is both entertaining and informative.