You answered your own question: if the macro in question is in a template, the macro may need to automatically create new documents based on the template. It's not unimaginable that a generated document might itself contain macros, for the same reason that any document already contains macros. Finally the macro language needs the ability to write these files to disk somehow. As for what kinds of files the macro should be able to use, I can think of several reasons why a macro might want to write a plain-text file, or even a binary file. Exporting spreadsheet data in some custom format required by a certain application comes to mind.
As for limiting the number of files created, this reduces the utility of the macro for users with larger needs than yours. You might be generating a mailing to thousands of customers and using a macro to do that.
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