Yeah, what Oliver said. To take that assembly point even farther - SU allows me to visualize not only the components for their fit but also think through the process of cutting and assembling. It also allows me to think though structural issues for furniture. I can run through building the project virtually. When I hit the shop I pretty much know each step.
Also, I use SU to try different design approaches to get the proportions right. Then I do a walk through/walk around with my wife so she gets input on the project before it gets too far. By getting her invested in the design process, she is much much more supportive of my shop addiction (but I can quit any time...).
Measure twice, cut once and CROSS OUT THE WRONG MARKS.