224 Valkyrie, 6.5 Gendel, 6.8 SPC, 300 AAC Blackout, 30 Remington AR, 350 Legend, 450 Bushmaster, 458 SOCOM, 50 Beowulf, etc, none of these relatively new cartridges do anything older cartridges can do balletically speaking. But what they do have in common and can do that many of the older cartridge of similar capability can't do it fit into the envelope require by the AR-15 platform. That may or may not interest you but when the 1994 AWB sunset in 2004 many in the firearms industry set out to show the anti-gun groups and the Fudds that the AR-15 plateform can do a lot more than 5.56. Like it or not the AR-15 is a huge hugely popular platform and the large variety of cartridges designed for the platform covering a huge variety of uses has helped extend that popularity.
I currently have ARs chambered in 5 different cartridges (6 if you count my can cannon
) and use them for very different things. A 45/70 in a lever-gun does not interest me much but 450 Bushmaster in a nice AR has been a lot of fun.