RTX cards don't render everything with ray tracing. They use it for light calculations, for example it can be used exactly for global illumination.
> why this is something we need.
The answer is simple. Because there is a limit to how realistic we can make the lighting and shadowing using the old models. We are almost at that limit. Using ray tracing removes that limit.
> challenging to do in a game with consistent frame rates
Try Quake II RTX. It is very consistent right off the bat, and if you turn on the resolution scaling, you can get completely consistent framerates (within 2-3 fps continuously). While it might not be a game with the most advanced graphics, it does have variety of scenes, and there is no issue with the framerates being "inconsistent".
"Need" is a strong word, but I'm sure someone will make a game where the shadows _matter_ for telling the story, or solving puzzles, or even just making the game world look more realistic and believable.
Or selling GFX cards..
I still don't understand the raytracing craze when it comes to games. Years ago when I wrote my first raytracer I might have got excited about it. But we have advanced so much with rasterization that I don't really understand why this is something we need.
Raytracing complexity is tricky and it is likely to prove challenging to do in a game with consistent frame rates. Soft shadows are expensive even for hardware.
I would be more excited about some other global illumination improvements, like photon mapping.