Originally posted by Doctor Q
Because PDF, now the standard file format, and Preview, now the standard viewer, are more general than the PICT files and SimpleText used in the good ol' days. In Preview, you can use the Export... menu choice to save your screenshot in one of a dozen formats, so you can have a PICT (or a Photoshop file or a jpeg) with a few extra clicks.
I don't think you can change the format used for screen captures. I did a little experimenting to try to find out: If you type screencapture and press RETURN in the Terminal window, you can see the options of the command-line method for doing screen captures. It lets you name the output file anything you like, but it seems to be a PDF file no matter what you pick. For example, the command screencapture peekaboo.jpg produces a file named peekaboo.jpg but it's actual still a PDF file, as the file command reveals:
doctorq% screencapture peekaboo.jpg
doctorq% file peekaboo.jpg
peekaboo.jpg: PDF document, version 1.3