If you have ever hassled with trying to install hinges on a non pre-hung door, you are familiar with the challenge. Figuring out the proper layout of the hinges on the door. Trying to take a single templet and align it perfectly straight up in order to prevent an off angle hinge. Trying to mount that templet at exactly the correct amount of depth to get a uniform edge. Routing without a glitch, or even moreso hand scribing an edge, then chiseling a flat base. Even if all of that goes perfectly, then we have the challenge of transferring those settings to the jamb in order to hang the door. How do we close the door on the hinge and scribe out the edge of the hinge onto the jamb. Opening the jamb and laying the hinges on it kinda works, but then how do you keep the distance correct for the clearance of let's say 1/8 inch at the top of the door. In plain words using the door with the hinges mounted is a rough way of getting the job done.