screen shot garmin c550

I can walk you through the How, can't say I see a good reason why.

from the main menu -> wrench -> display -> screen shot
from there you can enable or disable.
once enabled you will get a camera icon in the upper right corner, in the map mode the icon will be in the lower left. When you want a snapshot of any particular screen, press the camera icon. Connect the 550 to the pc, and look in the garmin folder for a folder named scrn inside is where the screen shots will be. They are the same size as the splash screen, so I suppose you could use it to make a splash screen. Or document a trip or if you find where a road is supposed to be, you could use it as an attachment when you fill out the Navteq report at http://mapreporter.navteq.com/dur-web-external/

Don't see anything real practical...yet

I used the screenshot feature on my nuvi 350 to take a snapshot of an incorrect map. I then used it to report the problem to Navteq. Other than for that or for documenting a trip or something I don't really see a point in it.