|GPU = Graphics Processing Unit. Although many people refer to video cards as GPUs, it's really the primary chip that's used on the graphics card. |
IGP = Integrated Graphics Processor. It's a GPU that's built into a motherboard. IGP's generally share the system memory & use the CPU to help display graphics.
I agree that a "high performance" card is of little value if you don't do anything that makes use of it, but adding an inexpensive dedicated video card can boost overall performance by eliminating the sharing of system memory & the CPU usage due to the integrated graphics.