4000k light bulbs

Why do the bulb companies make 4000k bulbs and call them "grow lights" and "great for growing"? Isn't that the spectrum that plants don't use? It seems like 6000k bulbs are in the minority.

4000k spectrum will grow plants just fine. Most prefer 3500k or 3000k for full cycle though. Gives a little more red spectrum the plants like in flower.