Properties files are used in numerous places since they allow you to change text attributes without recompiling code. Anytime you see text labels on
JSP pages or swing pages for example, such as the "Post A Replay" I see at the top of my page, its a good idea to extract those out to properties files rather than whats referred to as 'hard-coding' them. Hard-coding means that in order to change them, someone must recompile the code.
As you work in larger and larger systems, recompiling the code (performing a build) becomes a big process and re-doing an entire build to change one text label becomes cumbersome. It also allows non-developers to control text labels without having to edit or even read code.
In addition, it is also used to store settings that can be quickly and easily changed during runtime such as database login name or password.