I found Sketchup helps plan larger items where there are dimension constraints like a built-in for a closet or a workbench. Once you learn enough (and restrain yourself from becoming an "expert") of the tool it can be easier/faster to plan things out than pencil and paper. It could be that as a rookie, Sketchup lets me experiment with things where if I had a few projects under my belt of the same type I could just get rough dimensions jump in.

I do love heading into the shop and just making something though.