Lauchers are basically the software that launches applications when you touch them, and positions icons on the screen, and in the app tray (the apps you have at the bottom of your homescreen when you turn the phone on), and allows a little more customisation - for ex. changing the icons to certain colours etc.
It's basically a further step in customising things. I used to use one when i had all my icons the same colour. Good fun if, in your own words, you like to endlessly dick around with your phone!