A great feature to have—especially when dealing with client work—is to automatically hide the directory contents from public view for any directory that does not have an index page. (This is not the same as having a directory password-protected.)
If you have a domain address to an image (i.e. "mydomain.com/images/filename.jpg") and the "images" directory does not have an index file, a savvy surfer is be able to see all contents of the directory "images" by truncating the path name in a browser window. If they attempt to go to "mydomain.com/images/" in a browser, they would see the entire contents of the "images" folder. Worse, they could do this with any unprotected directory on your website.
Solution: Add "Options -Indexes" (without the double-quotes) to your .htaccess file. This will give any nosey visitor an error and stop them from viewing the guts of your website in a way you never intended.