Generally in a desktop, you won't notice RAM frequencies higher than 1600mhz. Passed that, very few programs benefit from it. In this scenario, the PC has a discrete graphics card with its own RAM, also known an VRAM. Graphics chips benefit greatly from higher frequencies, so VRAM tends to run at these higher frequencies. Techquickie on YouTube has a great explanation of this.
In an APU, you have a processor and graphics chip built into one piece of hardware. Since graphics chips benefit from higher frequency RAM, and desktop applications that are more CPU intensive don't really care in the grand scheme of things, it is better to have higher frequency RAM matched with APUs.
So basically if you have a CPU with a discrete video card, you won't really see a benefit from frequency as much as you will timings, but on an APU you will see a much larger performance gain from frequency.