The reasons are:

- I want to locate all implmenetations of the get_fcn() handler. By
  consistently using this macro, you can just grep for the macro and
  find them all.

- all implementations should follow the same style. This macro
  enforces the same names for arguments and avoids copy&paste.

- if we are going to add or change an argument, it becomes easier.
  That's because we can easily identify all implementation and can
  change arguments in one place.