Flashing Corners are an important part of window pan creation AND they help protect the rough openings from water intrusion.
When a heavy window is set into the rough opening, it always needs to be adjusted in place. It Never rests in the correct location immediately. To get the correct inside reveals, it will be moved to the right or left 1/2″ or so. The flashing corners provide protection from the Fish Mouth holes that will happen in the corners if Only Flexible SAF (Peel and stick) water proof membranes are used to create the lower pan for the window. The Rigid pre-formed flashing corners provide armor for these lower corners. See the photos below:
Additionally, when stucco is being applied to the wall, the flashing corners create great protection from furring nails and lath staples. If expansion metal is used, the flashing corners are even more important because the expansion metal always terminates at the corners of the windows. See photos below for examples.
The photos below show a stucco wall as it is prepared for the scratch coat of stucco. Notice the lath fasteners and the Expansion metal that terminates at the corners of the windows. The Expansion metal is nailed in place VERY close to our windows. This project used 4 Lightning Flash corners per window. The upper and lower corners were well protected from the penetrations by the installation of the lath wire and the expansion metal.