In most of the metro, a locked door is a problem with a fairly wide range of acceptable solutions, because the door itself is a modern manufactured item that can be repaired or swapped without anybody losing sleep. In the old parts of this town that is not true. A great deal of the building stock carries original timber doors, original frames, and ironmongery that has been in service for longer than anybody currently alive, and none of it can be reordered.
That changes the order of operations completely. The default becomes non-destructive by a wide margin, and the acceptable outcome is not merely that the door opens but that afterwards nothing about it looks different. Where a modern uPVC door might reasonably be drilled and re-fitted in twenty minutes, the equivalent decision on a hundred-and-fifty-year-old stile is one you do not get to reverse.
It also changes what can be fitted. Old timber has its own mind about moisture, sits in frames that were never square, and often carries mortices cut for hardware nobody manufactures now. A good deal of the lock repair work here is about keeping a period mechanism honest rather than replacing it, and where something genuinely has to change, the answer is usually a modern insert into the original casing rather than a new hole in old wood.