The reason is that CMYK represents a subtractive colour system, whereas the RGB (Red-Green-Blue) system is an additive colour system. RGB is used where you are constructing colours by mixing sources of light (eg Computer Monitors) and CMYK where you are constructing colours by mixing absorbing dyes (eg Printing).
The point is that the image you print out won't look the same as the one on your screen. Converting to CMYK ensures that the screen-image and the print-image will be approximately the same.
ps2pdfcmyk foobar.psThe output will be
foobar_cmyk.pdfViewing the PDF, you'll see the colours are somewhat less intense - this is CMYK.