Do I need the optional Wall Mount Plate

Almost every installation is unique, so its impossible to give a definitive answer.  There are three factors that may help you in deciding.

1. In almost every case,  the existing hole in the wall needs to be made about 1/4 inch taller for the touchscreen to properly recess inside the wall.  If the touch screen jams in the hole,  this is not a secure installation and the Velcro tape will not hold the touchscreen. The recess hole MUST allow the touchscreen to properly recess inside the wall regardless if you use the Wall Mount Plate or not.
2. In very rare installations,  the depth of the hollow wall just barely provides clearance when the touchscreen is recessed. In a normal installation, the rear of the touchscreen is recessed approximately 1 1/4 inch inside the wall.  Using a Wall Mount plate it will recess about 1 inch.
3. In some installations, the Velcro tape adhesive has pulled away from the wall covering.  In these cases,  the installer has created some type of frame to mount the touchscreen to.  These  are the installations that the Wall Mount Plate was developed,  to ensure a surface that the Velcro tape can bond to.