Can someone explain what is purpose of Windows RT?

I own Lumia 920 and HP Envy X2, my five year old desktop computer runs W8. This is best Windows version ever created (IMHO) containing most beautiful UI. I don't understand why people are negative about Metro. New Start Menu is so handy, clean, fast and good looking. Just customize it any way you want. But what about Windows RT. I recently bought HP Envy X2 and this device is blowing my mind. It contains full Windows 8 so I can run absolutely anything on it. Yeah, it is not the most powerful device, but I don't play games so I don't care. But before I bought this gadget, I tried some devices with RT. Honestly speaking, I have no idea why Microsoft even created this version. No possibility to run legacy apps is total fail. Tablet PC based on X86 chips and running full Windows 8 is great. This is future and Microsoft is on the right track, I have no doubt about that. But what is purpose to run Windows on ARM chip? Wouldn't it better to run one OS (maybe Windows Phone) on ARM based tablets and phones like Android/iOS and Windows only on x86 based Tablet PC. Now we have WP/RT/8 without any compatibility. Why there exist two different markets - WP store and W store? Why can't Microsoft just unify stores, one store for all devices and that all apps would be compatible with all devices. It would be even better that phones and tablets would be based on x86 chips so that in near future phones could run full Windows too. That would be complete unification of all devices. Just put your phone in a dock and you have full Windows experience. Maybe my dream is very close, just need to wait 2-3 years. Even today Lenovo K900 with dual-core Intel X86 chip completely destroys almost any ARM device: