In refractors, and certain other types of scope where you look through the end of the scope (rather than in the top/side with your Dob) a star diagonal diverts the light at 90° to make it more comfortable for the observer.
One is shown here in the third small picture:
http://www.bintel.com.au/Telescopes/...oductview.aspx
Imagine the scope was pointing nearly vertical. You'd need to be lying flat on the ground to see through it without a diagonal!