So what’s the difference between CMYK and RGB? CMYK vs. RGB at CreativePro has the quick answer.
For what it’s worth, I haven’t converted a file to CMYK in a long time. Since virtually everything I design ends up leaving my computer as a PDF/X-1a, I have InDesign convert to CMYK in the PDF-making process. You should note that this only works if you have a fully color managed workflow.