The traditional reason is if you are doing a heavy numerical simulation or not, but in practice I'm guessing it is down to the preference of whoever is in charg

The traditional reason is if you are doing a heavy numerical simulation or not, but in practice I'm guessing it is down to the preference of whoever is in charge. If you don't know Fortran, I wouldn't recommend you learn it to write a GUI app, but if you do know fortran and want to learn low level systems programming you can use it to write a GUI.