Engineering layouts change from Customer to Customer based on configuration schemes both interior and shipside.
One example, the Thai 747 has a longitudinal galley that runs the entire length of the RH
B-zone (between doors 2 and 3). The galley interfaces (ventilation, return, plumbing, electrical) do not all match existing/standard shipside locations. Also, interference with structure may drive the need for new vertical conduit locations to be created on different airlines configs.
Other monument physical locations such as partitions, lavs, closets, etc will force airlines to use blanking plates instead of having a window be exposed to the back side of a green colored primed honey-comb composite wall. When an airline installs a closet at a certain location, the airline will not install a sidewall, hence the need for blanking plates.
Finally, on some areas of the aircraft, there is just a blank half of sidewall adjacent to seat (no monument forcing a blanking plate). As people have previously mentioned, this is due to standard vertical conduit locations.