This is going to be a fairly long post, so bear with me... and Bruce or others, please step in to correct my errors...
A turbine engine generates power through heat-- as the fuel in the combustion chamber burns, it is extracted by the turbine section. The more power required, the more heat it generates. And, just like a normally aspirated piston engine, the higher the density altitude, the less power the engine has.
Unlike most normally aspirated piston engines, however, most modern turbine engines have an excess of power available. So, for example, the -5 on the 840 has the thermodynamic capability to generate 840 horsepower, but the airframe is limited to 717 horsepower. That limitation could be due to the gearbox, Vmc, general airframe limitations... but also it does provide better hot and high performance.
Thus, the turbine engine actually has two power indications-- horsepower (in the case of a -5), and Internal Turbine Temperature (ITT). A -10 uses different terminology, but the result is the same.
So on takeoff on a cool day at sea level, in a -5 840, you will easily be able to get 717 horsepower, and the ITT will be well below the limit (923). As the density altitude goes up, you will need more and more ITT to generate the same 717 horsepower, and eventually you won't be able to generate 717 horsepower at 923 degrees ITT. I don't have any -5 performance materials handy, but I would guess that happens around 6-7000 feet density altitude or so (it's been a long time since I flew a -5).
The more time you spend at high ITTs, the higher the wear on the turbine blades, which results in a little bit less power... which results in spending even more time at high ITTs... and so on. Eventually, they degrade enough that you really start to notice the decrease in available performance. It's a very slow process, but it does happen.
Now let's take a look at the -10. It uses "percentage of torque" instead of "horsepower" and "EGT" instead of "ITT" but the meaning is basically the same, though the actual numbers are not comparable.
The -10 on the 980 can generate 1,000 horsepower thermodynamically. However, you are still limited to basically 717 (733, to be specific). As a result, you are virtually never limited by the EGT on takeoff. In fact, you typically aren't limited by EGT on the 980 until FL200 or higher-- you are still getting the full 733 horsepower.
But wait, there's more! The -10's thermodynamic limit is actually much more conservative than the limit on the -5 in terms of wear on the turbine section. So running a -10 right on it's thermodynamic limit does not cause the wear on the turbine blades that running a -5 on it's limit does. And, in a virtuous circle, since the thermodynamic limits are so much higher on the -10 anyway, you spend much less time there.
The result is that -10s typically don't see the power degradation than -5s do over time.
If you are generally operating near sea level, then the extra power of the -10, while nice, may not be that important. However, in the case of hot and high operations, it makes a huge difference, both in terms of safety and long term cost.
