Obviously you can create a 2D sketch on a face or a plane, but did you know you can also select an edge? When you pre-select an edge and insert a sketch, SolidWorks will create a plane perpendicular to edge at the endpoint closest to where you select and open a sketch on it automatically. Two functions happen when you open a sketch like this. First it creates a reference plane and and second, it opens a sketch with the origin lined up on the endpoint of the selected edge.
This can be especially helpful for functions like miter flanges in sheet metal. When you click on the miter flange tool it will prompt you to select somewhere to insert a sketch. Using this technique is much easier than trying to select the thin face of the sheet metal part to insert a new sketch on. ~Lou