1) If you wanted to control the name of the uploaded file rather than just using the default name. some people prefer to rename the file to use a users name, or ID instead of the name of the file as it is.
2) The default file setting is a value that is given to the Server File Name binding if no file is uploaded.
the most common use of this is on the update page. on the update page, you create a hidden form element that contains the name of the file already stored in the database. you use the hidden element as the default file.
this way when a record is updated, but no new file is uploaded, the previous file name is retained.