When you are doing butt joints it is tricky to keep everything in position. You are holding the pieces in place, drilling holes and driving screws. The clamp shown in the picture above keeps everything in position beautifully.
The clamp has 2 fences. The base is made out of 3 layers of plywood. The middle and top layers are cut in half to create slots. The threaded rod fits into the middle slot. When you build the clamp, use spacers wrapped in wax paper to keep the slots the right size. The wax paper will keep the spacers from getting glued into position.