From Tracy Osborn via Smashing Mag:
“The power in paper sketching is the sketch’s ephemerality and how they feel less “real” than anything we create quickly on a computer. Start moving words and buttons around on a computer screen, and it’s tempting to fall into a certain direction and never explore alternate paths. Paper sketches force our imagination to fill in the gaps — far more quickly than if we added those details to a computer mockup.”
I keep a pad of graph paper on my desk. I use it for everything – scribbled notes, conceptual diagrams, little doodles, etc. Starting with paper is the best way to really grok something, before digitizing it for posterity and sharing.