The way to think about Design for Assembly is reducing the part count and minimizing the amount of time for assembly. Reducing both of those factors results in a more efficient design.
So you might ask: Why is it important to reduce part count? Reducing part count takes more effort than just designing the product the first way you design it. There are a lot of good reasons. One is the fewer parts you have, the fewer fixtures you need to measure the quality of them (i.e., Are they within spec or not within spec?). Also, every time you put together a part there is typically one type of fixture holding one part to the other one. If you don't have a part, you don't have to build and maintain that fixture.
You can view the full video and accompanying slides below.
Video by Williamson Visuals