MANY NOVICE DEVELOPERS struggle setting up Paths to files in scripts. Mike Duncan of Soliant sheds some light:
When importing data, it is useful to use variables to reference the file you are importing instead of using a full path that needs to be maintained. This allows you to better maintain your solution, and also the flexibility of referencing a dynamically named file in your scripted import.
One of the disadvantages of this is that you lose the ability to line up fields in the import dialog. Of course, since the variable is set only when you run the script, FileMaker has no knowledge of where the file is in order to line up the fields.