Why we needed high performance graphic cards for playing high quality games? And why we use extra graphic cards for high quality graphics like NVIDIA, ATI intead of using onboard graphics... Please explain me in detail....
ATI and nVidia have been developing and producing GPUs for a lot longer than Intel, which is responsible for most of the integrated stuff on motherboards these days. They've had longer to develop better hardware (and patents).
The first PC, the IBM 5150, did not have any onboard video at all. You were supposed to pick between a CGA or MDA card as an option when you purchased it. Later motherboards started including chipsets, and some of these chipsets included basic unaccelerated VGA video - sometimes with separate onboard basic VGA hardware (companies like S3, Cirrus Logic) or sometimes with this integrated in the chipset. Eventually chipsets started including basic 2D and even 3D acceleration - as Intel took over more and more of the chipset market, this has evolved into the GMA hardware included in Intel chipsets and CPUs now.
The point is, it's generally always been the case that if you wanted good graphics or video capabilities out of your PC, you needed to add that yourself.
Also, consider that a high-performance GPUs consume a lot of power and generate a lot of heat. Not everyone needs or wants a high-performance GPU (think office worker dealing mostly with Word and Excel documents).
There are a couple of reasons:
The GPU chip makers (NVIDIA and ATI -now AMD) have been working for a long time on specialized hardware for running games, therefore they have been able to put a lot of research and development into making cards that can run very intensive games at a very high resolutions at a reliable frame rate.
As a side note: Newer CPUs have placed the memory controller on the CPU itself. This has allowed Intel to jump into the integrated game by releasing 'decent' GPUs integrated onto the CPU itself. As long as you have enough system memory you can get decent quality because you dont have to run the connections over different bus's (AGP, PCI-E). This is different then the old integrated method where the GPU was on the motherboard and shared system memory. Intel's new Iris graphics is a indication that 'integrated' GPUs are getting a more viable option for gaming.