When I turned 12 my sisters taught me how to shave my legs while on vacation in Hawaii; this is a rite of passage for a lot of adolescent girls but in the swimming world, it's a pre-race ritual shared by both men and women. Who knows how long swimmers have been razoring-up before a big meet. Even Mark Spitz with his majestic 70s mustache and shaggy hair looked otherwise hairless when he swam for seven gold medals in 1972. Whenever it began, it's the norm now and I thought I'd look into why.
The first reason is the obvious one: cutting down on resistance in the water. There was a study done in the late 80s that sought to prove this point and it sort of did, by narrow margins. But if someone is just looking to gain a small advantage in the water (tenths or hundredths of a second) then it's worth the extra effort in the shower.
A swimmer's form is all about being able to glide through the water in the fastest, most efficient way they can. It only seems natural that a swimmer would do as much out the pool as possible to ultimately help their chances to win. Remember this amazing finish at the 2008 Beijing Olympics in the men's 4X100 meter freestyle relay? That American team won by less than a tenth of a second. In swimming, those fractions of a moment can mean the difference between gold and silver.
A large reason that swimmers make sure they shave before a race is because of something you probably hadn't thought of: to increase sensation in the water. I know after I shave my legs, I love the feeling of getting into my bed at night because I feel the sheets so much more than I do when I've gone (ahem) a few days between shaving. Same goes for swimmers; they have a heightened sense of the water when they have shaved because it removes that fine layer of dead skill cells that often build up on our skin in addition to removing unwanted hair. The skin underneath is much more sensitive and apparently helps swimmers perform at a higher level during races.
So why do swimmers shave? So they can go faster, of course! And I don't think the ladies mind it either.