If you've ever applied caulk around the edge of a bathtub, you know how difficult it is to produce a smooth, even bead. You need a steady hand, constant tube speed, and even pressure on the trigger. Inevitably, you need the help of a Popsicle stick, putty knife or the ever-practical finger.
Now imagine trying to lay a bead of flange sealant along the edge of a car door. The height, width and location of the bead must be accurate to within a millimeter. If not enough sealant is applied, or if it's applied in the wrong spot, you'll get an inadequate seal. Applying too much wastes material and results in extra work to clean up the overflow. If that's not challenging enough, you also have to finish one door every 40 seconds.