Here's why I use it
Convenience - I can do 100% of what I need a computer to do wherever I want just by carrying around something the size of a legal pad. I don't need a backpack to carry everything in, I don't need to swap back and forth between a tablet to do netbook stuff and a laptop to run x86 programs. Same device I use as an eReader also runs Matlab, Ableton, and Wildstar.
PDF Markups - I import PDF's from the cloud or thumbdrive, markup or do redlines in Drawboard with the stylus then export. Without this it would either be draw on hard copies and rescan them, or use some BT solution that's a hassle and not as accurate
Digital Notetaking - Studying for a licensing exam right now, every weekend I'm at Starbucks with my SP3 and a textbook doing problems. When I go to class, I just lay this thing flat on the desk and take notes into OneNote. The fact it's digital is huge. I can lasso to move stuff around, sketch diagrams, convert to text/equation, auto sync to the cloud.
OneNote - this thing is the killer app for the SP and I haven't seen anything that comes as close to being as useful. Any type of info I need digitized goes in here. To Do Lists, Important Documents, Manuals, etc. Buying a house right now, I have a notebook containing all my design center and loan documents.