Does anyone know the actual reason you're required to file a flight
plan when crossing the boarding between the U.S. and Canada? Does
someone sit down at the end of the year and try to count operations
based on them? They carry NO customs benefit since customs are handled
separately.

Usually, when we fly south into Mexico, we don't both filing since the
U.S. has no way to actually hand the flight plan to Mexico. Most of
the Mexico AOI don't even have working phones, much less computers.
However, they are always happy to see you arrive at any time. Of
course crossing the ADIZ northbound from Mexico is a different thing
all together.

-Robert