Advantages: provides support for your testicles and keeps your penis in a more comfortable position while you swim. Allows you to use your legs better when you swim, and doesn't fill up with air like normal trunks.
Disadvantages: shows off your whole body and only covers your privates, which makes it very disgusting to see old fat hairy men wearing them. Also shows off the general size of your private parts, which can be embarrassing if you have a very small package, because everyone would see your small bulge. A speedo without a decent layer of lining in the front will also make it very obvious if a guy's penis is circumcised.
For my high school swim team, we all wear speedoes, so girls came to watch our swim meets to check out the bodies and the packages. But that makes Johnny embarrassed, because his package is small and has basically no bulge in his speedo.
Speedos are quite a common swimwear choice for men and boys who live in Europe, South America, Australia, and Asia (especially Russia). Unfortunately, for some bizarre reason, speedos aren't well received in North America despite them being sexier in style than conventional swimming shorts.
Speedos have even managed to make their way into rules and laws of certain establishments and areas. There are many swimming pools in Europe (especially France), which require all males to wear speedos for sanitary purposes. The reason is because males may be in dirty areas outside the pool facility while wearing conventional swimming shorts and then track the filth into the pool; but it is assumed that with speedos, a pair of pants is worn over them until it's time for swimming, thus making speedos less exposed to the elements than regular shorts.
On the opposite end of the spectrum, the town of Cape May New Jersey USA had banned men from wearing speedos in public. In the 1960's, the South Jersey beach town made a law stating that any male over 12 years of age could not wear swim briefs. This ban was passed mainly because of homophobic claims that speedos were essentially worn by gay men for no other reason but to advertise their 'goods' to each other. The ban was finally lifted in April 2005, more than three decades later.
To sum up, speedos are the sexiest swim attire for men and the world needs more of them.
Girl: So do I! nice penis!
Girl: No problem want to have sex.
Boy: Hell ya!!!( gets on floor and takes off pants)
Girl: Yes!!!(takes off all clothes and lays on guy)
Swimmer: speedo or TYR
Non swimmer: HAHAHA! you wear speedos?!