Why do they do this? I thought the point of the sport was to see who would get across the pool under their own power and without a tool. It seems to defeat the purpose of the competition when they include arbitrary rules like this.
You could liken it to the difference between running and walking races. Without the rules, a walking racer could discover an amazing new technique that greatly increases their speed - i.e. running. Or long jump vs. triple jump. Both are 'how far can you get across the sand without disturbing it'. you could get amazing triple jump distances if you could just keep running to the edge of the pit, then jump.
Even so, you could describe the point as who could get across the surface of the pool under their own power, and that there is a permitted amount of underwater time or distance due to the nature of turns and dives. Much like in a walking race, you can 'run' a limited number of steps, otherwise half the field would likely be disqualified in most races.
Butterfly was originally a breaststroke variant, as it complied with the then rules for that stroke. The rules for breaststroke were changed and a new event came about. Conversely, English Backstroke has simply vanished in favour of back crawl.
All sports include arbitrary rules to try to make them more fun for both participants and viewers. Basketball isn't just about seeing who can get a ball through the hoop the most times, car racing isn't just about who can be the first N times around a track etc.
But honestly, it annoys me when things like swimming and car racing bind themselves up with rules that amount to "no, do it the conventional way, not the better way". It should be legit to win by innovation.
g-forces causing harm to drivers: https://what-if.xkcd.com/116/
Smaller events also have to allow the competitors to focus on a smaller range of capabilities so they can actually find competent players. Robot and drone competitions come to mind where the equipment is often standardized so they're only testing skills or programming, not being-the-best-at-everything which would end up as "throwing the most money at it".
The official reason? Safety concerns (from oxygen deprivation) and the fact that people swimming underwater makes for a less spectator friendly sport. There might be some other unofficial reasons, but not sure what they might be. My guess is that the spectator angle is the main reason.
> After the 1956 Olympics, in which Japan's Masaru Furukawa swam almost all of the breaststroke underwater, FINA limited how long a breaststroker could swim underwater. One concern (beyond the obvious downside for spectators) was safety: As others adopted Furukawa's technique, they became worried about swimmers passing out from oxygen deprivation.
> In the 1988 Seoul Olympics, Berkoff of the U.S. and another Olympian, Japan's Daichi Suzuki, used extended underwater dolphin kicks to dominate the backstroke. That prompted FINA to enact a rule that competitors could only swim underwater for 10 meters. A year later, the allowed distance was increased to 15 meters.
"In the mid 50s Miguel and the pitcher of the Basque Bar, Félix Erausquin invented how to adapt that instrument to throw the javelin. In the manner of the disk or hammer, pitcher turns on itself before throwing. After much training, Miguel de la Quadra-Salcedo presented the "Spanish style" of throwing the javelin. The effectiveness was that high that in 1956 added 20 meters to the world record, which at the time was around 80. However, the record was never approved by the IAAF (International Association of Athletics Federations). It thought that launch was too dangerous for the public because during the rotation the javelin tip was focused to them." [1]
https://en.wikipedia.org/wiki/Miguel_de_la_Quadra-Salcedo#Ea...
edit: a couple more links, including a video [2], [3]
[2] http://rethinkingathletics.blogspot.com/2013/12/the-javelin-...
Without rules, we'd swimming would be people canoes racing back and forth. But this particular rule is health-related. Breath-holding is not a healthy thing to do day after day, especially with groups of young kids. They aren't going to drown but their brains aren't going to thank them.
(Unrestrained drowning is actually very hard thing for a healthy person to do in a pool. Most competitive swimmers have experienced the adrenaline and panic of inhaling a large amount of water. It happened to me during a scuba lesson. During a mid-water CPR exercise I inhaled a lung full of water. They called me aquaman after seeing me leap, with tank, onto the deck like Kevin Costner in waterworld.)