The plug is designed to close the open cavity at the bottom of the grip, not allowing debri to enter and reach trigger assembly and firing pin. It also fits flush to eliminate snagging on open cavity. Used by LE & military worldwide. Does not fit Gen4 models.
I looked over a few other brands of plugs that fit my Glocks and purchased the Jentra. There was no reason to spend more money on another brand just because the name was more popular. Insert the Jentra plug partially into the backstrap space and slightly depress the plastic tab while pushing the plug inward. The plug snaps into place without the use of tools. A ballpoint pen with retracted ink cartridge will work just fine. The color matched well and the gap is sealed from dirt or debris.