This feature is made possible largely as a function of the hull designs of these boats (broad, flat, shallow stern sections), but I would say the purpose is primarily two-fold: weight savings and self-draining.
When you think about it, the transom on a more conventional boat is never in the water, so it's not helping to make the boat float. In many cases, the transom simply provides a back wall to the cockpit area. That's handy for comfort, safety, keeping things aboard, and for providing a place to mount stuff. But, it's not a necessity for floating the boat.
And the same transom area that forms a back wall to the cockpit and contains everything, also contains any water that ships aboard. So conventional boats need cockpit scuppers/drains to get the water out.
These racing cockpits that you find on the sport boats do away with the transom and just leave the aft end of the cockpit completely open, with the cockpit sole typically only a half foot or so above the waterline. You might think a lot of water would ship aboard over the stern, but it really doesn't. And of course, any water that comes in from the forward end of the boat, just flows right out the back. Very impressive self-bailing.
It also results in a roomier cockpit for working the race boat.
I remember when I was towing the Melges 24 back from the factory in Zenda Wisconsin (1993), we had pulled into a gas station out in Ohio or western PA. Delivery of the the carbon mast had been delayed, so we only had the hull on the trailer. A couple locals came over to look at the boat, and one of them asked (with a drawlish accent) "What is that thang? Some kind of ex-per-i-men-tal sub-ma-rine?" When I explained that it was a sailboat, he didn't believe that it would float without a back end.
It's not a transom arrangement you'd ever want in a cruising boat, but it has its place on sport boats.