When you have the print queue window open for a printer in Mac OS X Leopard, the Printer menu > Print Test Page command is active. That command sends the CUPS test page to the selected print queue.
The test page file itself is a PostScript document located at this path:
/usr/share/cups/data/testprint.ps
Since Leopard — like Mac OS X versions since Panther, has a built-in PostScript interpreter — you can open up this PostScript document and have it rendered as a PDF. By default, it will be sent to the Preview application.
It’s helpful to be able to view this document on-screen, so you can see what it is supposed to look like, in case your printer is not working as intended. You can compare the on-screen results with the printed output to verify how well your printer is working.