You are welcome! Actually, there are potentially several reasons you might still want/need to use FTP after you have installed Joomla!:
1) You might want/need to upload (for subsequent linking) a file larger than the 8 MB that DH's PHP (under which Joomla! runs) allows - for instance, a largish (10 MB, for instance?) .PDF or media file. Note that neither Joomla!'s Media Manager, nor the WebFTP program available on DH will let you do this (it also runs on PHP and has that upload file size limit).
2) You may want to upload "many" images, or other files in a quick and efficient manner - the Joomla Media Manager allows you to upload a single file at a time while an FTP client will let you upload whole bunches of files at once.
3) When/if you need to manually upgrade, or in some other way manipulate, your Joomla! installation (files, data, etc.) - maybe a backup, for instance? - you will need to use FTP (to upload groups of changed files and/or download copies of everything for safe backup purposes - again, find it is much easier and faster to use a "real" FTP client than the DH WebFTP installation.
There are probably more, but these three immediately come to mind, and are each "real world" situations you are likely to encounter.