Why do image scanners take a while to start doing their work?

Yeah… when I press scan it takes a few tens of seconds before the scanning head starts moving. I wonder why that is. If it was the printer, I’d still forgive it because of the buffer possibility. But the scanner? What’s the issue?

By the way, this is an HP Scanjet 3300c under Mac OS X, but I had this same issue when scanning on Windows, as far as I remember.