Many woodworkers don't have a large enough clamping table for doors and other wide objects. And, bar clamps have a tendency to flop to one side when you lay them on a flat surface.
woodworking technique shop tip

Summon a couple of sawhorses to the rescue. Notch the horses and insert your bar clamps or pipe clamps, as shown in the drawing. For flat sawhorses and workbenches, notch two 2x4s in a similar fashion and tack them to the work surface.
—Rob Huffman, Danville, Va.