For people who don't understand why anyone would ever want to wear a jockstrap, some of the history in this thread is useful, but a few other points may also help.
First, you will have read from the Wikipedia site that even before jocks there was a similar garment to wear for "modesty's" sake under the long, woolen bathing suits that men wore, from neck to knee. Since these suits were not lined, everything could show, and that was considered immodest. After all, men were wearing a nearly head to toe garment to avoid even having their chests visible.
The actual jock strap, short for jockey strap, had to do with bicycle jockeys, that is, bicycle riders, who found it comfortable to have their genitals tucked up against their bodies when riding bikes with really poor suspension and flex on typical rough roads. As it happens, a jock is still good for this purpose today, if you're not wearing a high tech pair of bike shorts. If I'm wearing my usual summer baggy shorts and, maybe, loose thin boxers under, putting a jock on can make a ride more comfortable with less danger of things getting caught, unexpectedly, against the seat.
But in school gyms and on basketball courts and among swimmers and runners, jocks were very common from the 60's until fairly recently. Again two reasons. One major one is that gym shorts that guys wore as part of school expectations were quite short. Same with jogging shorts, basketball shorts and bathing trunks. All these had fairly short legs below the crotch, and they often had slits on the outside of the legs to provide more flexibility as you were running. If you wore nothing under them, you would likely have your penis or nuts on display part of the time, and, at that time, this was kind of shocking or embarrassing. Things are different now, but that was then. Also, if your dick or balls are bouncing around, as on a long run, your nuts, at least could get kind of sore. Still true now. Guys could wear Y type briefs under their shorts and that would address the gym short or running shorts problem, but was a kind of wet mess under swim trunks, which often had a liner that made wearing anything underneath, including a jock, unnecessary. The jock, however, was the common thing to wear. Everyone did, and one advantage was that the thing stayed in the locker until your next workout, while your underwear, if you wore it during sports, would be wet and clinging to your just showered body for the rest of the day, possibly sweat soaked. Maybe not a great idea unless your colleagues or your girl really liked the sweaty smell.
So now, with modern fibers like microfiber and polyester, you can have briefs, or compression shorts, or boxer briefs, or the great UnderArmor boxer jock, wear it under your workout gear (which probably hangs down to your knees anyway) and be perfectly comfortable. You can leave it in the locker until the next time you need it. And, with shorts down to your knees, going commando works too, unless your junk bounces too much jogging on a treadmill or a stationary bike and things get uncomfortable.
So why does a jock have no backside? I don't think, from the sports view, this has anything to do with showing off your ass. You always wear something on top of a jock when playing sports or working out. But the lack of a backside is a bit of an issue for someone putting a jock on for the first time. The straps feel strange. But why no back? I think it's because, at the time they were first designed, there was no comfortable thin stretchy material that you could wear over your butt, and when you were running, pivoting, jumping or whatever, you didn't want extra stuff pulling tight against your your butt and hips, like pants that are too small do. At the same time, you didn't want to be playing ball with cotton underwear briefs under your short that were soaking with sweat. And you didn't need extra clothes that wouldn't dry for hours under your quick drying swim suit. The original design let the jock do it's job -- keep your stuff tucked in comfortably, and out of sight -- without dressing you in another sweaty layer.
So why would anyone wear this thing today? Well, in sports or working out, it's just a matter of comfort, and jocks can be substituted with all manner of underwear. It all works fine, and the high tech fabric types, like with UnderArmor, make a true jock unnecessary. Unnecessary but optional, and a fine choice.
The other feature of jocks, very erotic for many, has to do with people who enjoy wearing them to be seen, with eye catching colors and designs. These guys are enjoying them as garments for show. And, for many men, seeing a man in a jock is something of a turn on. But that's been discussed in other entries.
Hope this helps...