It might seem like a waste of time to draw what you are just going to piece together on the computer later. I personally have three reasons for sketching layouts for my print design work.
Three reasons for sketching my print design layouts:
For a trifold brochure, I print a trifold brochure sketch guide that I created for myself. I then run a quick sketch over that getting my ideas down on paper as quickly as possible. I loosely use this rough sketch as a guide for working in Photoshop and InDesign. It only takes a few minutes, but aids collecting my thoughts for these projects.
Here is what one of my sketches look like: