Psst! is an application that you probably don't need right now but which you might need at some point. But at that point, it will be too late to get it.
Psst! lets you change the volume of the Startup Chime. (The sound that your Mac makes when you start it up.) You can just turn the volume down, or get rid of it altogether. If you use your Mac in the library or in a quiet house, this will come in handy. Install Psst! now and it will just run in the background until you need it.