What does "255" mean? If you just take it to mean "max saturation that your display can output" then it's going to be perceived as a different color depending on the viewer's display. That is undesirable. And due to historical reasons (the capabilities of CRT monitors) the meaning of "max saturation" was de facto standardized to what we now call sRGB. If you want to encode a greater range of colors than sRGB then you need to be able to say that hey, this "255" is more saturated than what sRGB considers "255" and should not be displayed on sRGB without conversion.