Hubby's hands are the size of baseball mitts, so it is no problem for him to hold two 2x4 boards in one hand and a drill in the other hand, AND apply enough pressure on those two boards to hold them tightly together while he drills a screw into them.
I can't do that. My hands are too small. If I had three hands I could hold both 2x4s AND drill. But I overcome this deficiency by using clamps. In this project, I could have used four hands. So I used a clamp on one side to hold the boards together, and a clamp on the other side to hold the edges flush. I also had to hook two clamps together to extend the clamp length. That left my hands free to put the screws in quickly and easily.