. converting and doing math with dates and times
. reading / writing XML or HTML files
. using regular expressions
. accessing operating system and using its feature, like e.g. reading / writing files or issuing system commands
. web / html / cgi support ( to write web applications )
. support to implement GUIs
. support to work with databases like mySQL or SQLite
... and many many more.
Thus in that sense they can be seen as extensions or add-ons of native Python.
In a more general sense whenever you write re-usable code / functions you might want to implement those as modules for easy re-use by yourself or others. See http://docs.python.org/2/tutorial/modules.html for a more sophisticated explanation ...