Blue is the B in RGB. It’s an essential wavelength used by RBG or RGBW (which includes white pixels) displays in order to accurately reproduce the various colors we want to look at. They are carefully blended together to reproduce billions of different colors.
Blue light has a higher frequency than red or green light, so prolonged exposure can be fatiguing on the eyes. But this usually requires many, many hours of screen time over many days - which I suppose is quite common for a lot of people. There are other factors which arguably contribute more to eye strain though such as uneven backlight strobing which can be an issue for lower quality displays.
The reason you can’t just turn the blue light off is you wouldn’t be able to accurately produce a ton of colors. Even if you aren’t necessarily viewing something with a ton of blue color in it at the time, removing all blue light from the equation would alter most of the other colors that you ARE looking at. There are software solutions such as f.lux that try to reduce strain by lowering blue light and compensate by raising “gentler” wavelengths, but they produce a visibly warmer, more yellowed effect which can be less than ideal in some scenarios.