Mag plates, or wrap around plates, are useful in numerous situations. Consider the unfortunate soul whose house has been broken into, or at least was attempted. The rapscallions may have used a crowbar or something to attempt to pry the door open or pry the lock off the door, leaving unsightly marks in the wood on the door and the door jamb. The marks on the door can be covered using a mag plate.
The mag plate covers up unsightly marks, but it also serves to reinforce the door. If we consider the door that was molested by neighborly urban youths, it may have lost some of its structural integrity during its trials. Structurally, the mag plate can reinforce the door in a retrofit situation as well.
Lots of old houses have what are called mortise locks, and they are generally really robust. They may last over 100 years with minimal upkeep and lubrication! Unfortunately, 100 years after they were installed it may be difficult to find parts for them, and they like other kinetic mechanical systems will eventually fail. The difficulty in procuring replacement parts means that it may well be less expensive to do a retrofit on the door than to fix the mortise lock. Mortise locks were constructed with few standards and the placement of various features on the lock will be different than other mortise locks.
Putting a mag plate over the holes left by the mortise lock not only covers up unsightly holes, but reinforces the structural integrity of the door. Without the mortise lock in the door there will be a large cavity, leaving perhaps 1/4 inch of wood on either side of the cavity. A swift punch or kick near this spot and the door will quickly yield a large enough hole that the perpetrator might be able to reach the deadbolt above the hole and unlock it. If there is a plate covering this spot, it will be nearly impossible for our perpetrator to force their way in.
Mag plates are pretty easy to install most of the time. You need only take the lock off the door with a Phillips screwdriver, put the mag plate over the door so the holes line up, and put the lock back on the same way you took it off. Make sure the door shuts because if the fit is tight you may need to get out your chisel and remove some wood from underneath the mag plate. You may consider calling me at this point if you are inexperienced with a chisel. If the door does close and the lock appears to be working properly, good job! You can tighten everything up, put the four screws on the mag plate, and reflect on a job well done.
Pitfalls that may occur: Make sure you get the correct size of mag plate. This requires measuring the thickness of the door, and measuring how far from the edge of the door the center of the lock is. Also decide what color you want the mag plate to be. When you go to the store, whether on the internet or in the real world, it may help if you have a picture of the door with you. Consider getting a mag plate the same color as the lock it will go under.
Secondly, make sure the mag plate is on the door tight. If it doesn’t line up with the holes perfectly when it is on tight, you may need to redrill the holes.
Third, when you install it if you are also drilling the holes, measure twice. Make sure the plate is in the correct spot and covering up everything you need it to before you drill giant holes in your door.