As a computer programmer with over 30 years experience, I would guess that "pc mapping" will record a gps point when you tell it to, like after you found something. You would then dump the "pc mapping" log to your computer at home where you can merge the data with maps, record notes about what your found, etc.
I would think the processor inside the ctx will be to busy doing its thing to display maps and such like other users have posted. Besides the size of the screen would really limit what could be displayed.
For me, the gps would really be useful to record when I found something of interest. This would be very useful in keeping a detecting diary. Besides, you can do so many more things on your home pc with gps data than you could do on the ctx unit itself.
As for the color display, you increase the amount of information you give to the user by using colors. I'm sure the minelab programmers will find some interesting ways to use the color display to tell us what the ctx is seeing underground.