Here's the problem. RGB handles pretty much any color. CMYK, on the other hand, while it will handle any color on your computer, when it's sent to a printer that uses the CMYK print process, those colors may change as CMYK printing uses a very specific color profile. So, if you're making a graphic which is intended to be printed, you use CMYK so that the colors you create in the graphic will print accurately as you created them.

Talent is God's gift to you. Using it is your gift to God.