Many of the new cartridges dont necessarily differ as much as the associating specs for the cartridge i.e. the standard twist rate and accompanying bullets. A .243 Win with the right powder will run with or slightly outperform a 6mm Creedmoor but the creed will push ahead due to its heavy for caliber bullets and faster twist.
Its all about efficiency. Burning the least amount of powder possible for a desired effect. Its also just simple business economics. You want people to buy your new rifle? Sell em on a new cartridge too! This generates sales and boosts everything that comes with it. People get bored with their stale old .30-06's or .270's. Everyone likes to get the latest and greatest, its the American way.
I know a lot of people who switched to a 6.5 Creedmoor when the hype was enormous. It really is a good round for our area of PA, not too many shots are over 100 yards and its mild recoil make it a pretty good little deer cartridge. The fact that a box of ammo that used to be $22-$25 is now $60-$100 shows that theres a huge demand for it.
I personally like oddball stuff or stuff that I know works. I dont really care what a company claims, if it is intriguing or proven to work for my purpose, I'll build or buy one and try it.
I like that people are innovating and as technology advances and we learn, stuff does get better. Like car engines, my 06 Subaru gets 30+ mpg average. 40 years ago, that was unheard of and now, thats not even that impressive.
The 6.8 Western is the newest kid on the block, catering to the short, fast magnum crowd. Soon people will dump the 6.5 PRC and go get a 6.8 Western. These new cartridges also breed better components and rifles. Hopefully we will see more .277" bullets on the market too. The biggest reason for its inception is that .270 Win lacks a fast enough twist to stabilize a heavier .277 bullet so a new cartridge was born. Good move too, Remington found out more than once that improving a standard cartridge can confuse regular people who dont understand it. The 7mm Express and .244 Rem come to mind.