What is the use for the input file name?

Some of Livecycle function need inputfilename besides the document object, such as following. The document object already give as a input parameter, why still need inputfilename? Actually I use some other dummy name that is different from the physical file name, it still works. So just do not get it why need input file name parameter in some Adobe LiveCycle class functions?

CreatePDFResult createPDF(Document inDoc, String inputFileName, String pdfSettings, String securitySettings, Document inSettingsDoc, Document inXMPDoc)

Thanks for your help,
P