Valuable in explaining to people how to do different stuff in applications, or just sort of generally cool.
Also, valuable when designing web pages as a person with one computer, so your Windows friend can send you screenshots of what stuff looks like in IE (or, if you're on the dark side, so your Mac friend can send pics of what it looks like in Safari)....
Person 2: Nah, just email me a screenshot.
Rachael:"Yeah, shes a complete screen shot."
- (verb): "God, you look so good in your new glasses! I'd totally screenshot the shit out of you right now!"
Good screen shot, man. You hit Tom Selleck right in the face!