I wonder if this feature came about not because it was originally in the "requirements", but because the layout of the hardware key matrix/wire routing may have not been decided completely, and it was easier to include several selectable lookup tables in the masked ROM than have to change the mask (at great cost) later. Then someone thought of adding the ability to change it at runtime, perhaps for testing, and the rest is history...
...and in case the alternate lookup tables don't seem to map to any keyboard layout, here's an example showing that a typical keyboard more often than not does not have the rows and columns in "physical" order: http://map.grauw.nl/articles/keymatrix.php
...and in case the alternate lookup tables don't seem to map to any keyboard layout, here's an example showing that a typical keyboard more often than not does not have the rows and columns in "physical" order: http://map.grauw.nl/articles/keymatrix.php