Pushing the shoulder and neck back allows more room for longer ogive bullets within the same mag length (an example - we can't load 75 ELD/A-max's in 223/5.56 at mag length because the ogive has to be seated down into the case neck, BUT 22 ARC can use this bullet without issue). Moving the neck and shoulder back so also can move the neck/shoulder junction farther back to align with the base (or bearing shank/boattail junction) of bullets for a given cartridge length, which reduces lateral forces on the base of the bullet during primary ignition. Blowing out body taper and using steeper shoulder angles helps give back some of the powder capacity lost by pushing back the shoulder. A prime example of these differences in practice is the 300 PRC or 300 Win Mag - We shortened the case and pushed back the shoulder AND made the neck longer so we could improve a LOT of shortcomings of the 300WM, and resultingly, the PRC is easier to tune than the WM. Another example is the 6 ARC - we had 6 Grendel wildcats for almost 2 decades before the 6 ARC came out, but we couldn't get long ogive, heavy, high BC bullets into mag length with the full length Grendel case (I can load 105 Hybrids in my 243LBC/6 Grendel, but can't load 108 or 109ELD's or 110 A-tips). So there are a LOT of reasons we see so many "new" cartridges being released which improve upon a similar but older case design (not to be confused with "Ackley Improved" vernacular).
In many instances, no, these advantages aren't enough to ever warrant someone trade in their old rifle - BUT - there are 3,000 Americans turning 18yrs old every day, becoming of age to legally purchase longguns, and there are over a million new firearms being sold every month, so it should be quite obvious that there is plenty of market and plenty of room for new cartridges which offer tangible advantages over older designs. Personally, I tend to buy/build at least one new rifle every year, even though I would tell you that I have what I need, and have had what I've wanted enough to just not really want many/any more, so I'm certain I'll own at least 20 new/different cartridges in the future which I don't currently or haven't ever loaded in the past... Frankly, I'm absolutely stoked about a 25GT or 25 Creedmoor, and plan to convert one of my rifles in the near future, and I have a 37" blank on my shelf with a .416" groove diameter which will be a Hellfire or Stroker in the next ~18mos... Probably mix in either a 300 Norma Improved or a 338 Lapua Improved in the next couple of years also... I've had 9 F-150's so far, and I'll buy another in the next 2 years, and my wife will probably get a new Explorer shortly thereafter... I'll probably have around 40 more pairs of boots and 100 more pairs of running shoes before my time is up... Not sure I'd get mad about any of those choices if anyone else made them...