customisation really. I have my home screen super minimal, clock and a few buttons. Launchers provide different ways of grouping things, so Microsoft launcher has "pages", widget page which scrolls vertically, a notes page, recent contacts etc etc, and they usually change the app drawer (and some allow you to change app icons on the app drawer - buzz launcher doesnt!)
Arrow Launcher also has an iphone esque "swipe up" drawer, to quick change settings / display brightness etc.
Also, with buzz launcher I set the homescreen grid to 12x12 and have super tiny icons.
They are usually (the good ones) faster than a stock launcher and usually have some nifty features.