It is purely an FSX thing. Nothing to do with P3D. Principally it's do with how FSX utilises Microsofts DirectX graphics engine. Generally DirectX has improved graphics in Windows over the years and the higher the number the more eye-candy can be seen in gaming, graphics, etc. When FSX was being finalised DirectX9 was the engine in use. Direct10 was just round the corner. We are now (2017) at DirectX 12.
If you're happy about the way your FSX looks and feels then you don't need it. If, like many of us you've ever thought to yourself "I wish FSX looked more real, better, less cartoony, less, less like FSX and more like flying an aeroplane" then read on.
In your FSX Menu Graphics settings you'll see a tick box saying DX 10 PREVIEW MODE. This was ACES studio's plan to integrate DX10 compatibility into FSX. They never got round to it before releasing the final version of FSX Deluxe + Acceleration. Consequentially they left crucial code out of the core program and FSX does not work in DX10 Preview Mode. Well, try it yourself, put a tick in the box and take a flight and you'll see it looks awful, textures missing, big black squares where lights are meant to be, stuttering frame rates and all in all, a mess.
Enter Steve,
steve's DX10 Fixer is a standalone program that amongst other things makes changes to your fsx.cfg file to allow FSX to successfully run in DX10 Mode with textures and shaders, all corrected and optimised.
Steve's DX10 Fixer uses shaders and code to unleash what FSX would have looked like if Aces had finished making FSX DX10 compliant. Lighting is better, surfaces reflect light correctly. Cockpit shadows scuttle across gauges and switches on the panel as you bank and roll, the skies look better, clouds, etc
I've been running steve's DX10 Fixer for about 6 months and I have to say that when, on the very odd occasion I run in DX9 mode (usually to check something) my immediate reaction on loading a free fight is "Ughh, that's ugly!"
And it's not just there for the eye-candy either. The FPS is much improved over DX9 and stutters and tearing also all but eliminated. There's a lot going on behind the scenes to make the sim a lot more stable and solid feeling.
In all it's a lot of little differences that all add up to one massive improvement in the In-sim experience.
Installing is easy - just use the .exe and leave everything at default. There is a manual - read it - to make further tweaks and improvements according to how you've got your sim set up. Steve always replies to support questions and is deeply knowledgeable about all things FSX. He is often cited as one of the most important people in the sim community.
Flicking back and forth between DX10 and DX9 is as easy as clicking a switch in the Fixer program on your desktop. If you run nVidia Inspector to manage your graphics card Anti-aliasing settings, etc you can store two profiles to drop into the Inspector when you switch over. I have one profile named dx9 and the other dx10. Simples. Once it's all set up to your liking the Fixer remembers your last settings and you can forget about it. FSX just loads normally with no other actions required. I haven't touched the Fixer program for weeks, months now actually. Oh, and FSX loads quicker than it used to under DX9.
Sorry I've ranted, there's lots of info out there and Steve has his own support page on avsim. I do not work for him nor am I affiliated with him in any way, shape or form. I'm just a very satisfied customer.