they are for quickly moving your mouse pointer into a corner with a pre-defined action that you set up in System Prefs. basically instead of triggering Expose with a keyboard command you can do it by moving the pointer into the corner. i love them and can't live without 'em.
I've got screensaver on top left, expose (F9) on top right, desktop (F11) on bottom right and dashboard on bottom left, I didn't use hot corners the first few months, but now I couldn't live withouth them
I dont seem to have any use for the corners. The side buttons on my mightymouse activate 'all windows', the clickwheel button activates 'desktop', dashboard is in the dock and I dont use a screensaver.
I used to use them when I took my laptop out and didnt have a mouse though.
I use command and touch a side of my screen to flip spaces in virtuedesktops though.