lets you keep track of the celestial time. The Sidereal Clock calculates the local mean sidereal time (LMST) based upon the computer's local clock, your time zone, and your west longitude.
Astronomy buffs use this type of clock which is based upon "star time". If you used your normal clock and timed when a distant star passed due south of you each night, you would find that each night it passed about four minutes earlier. Thus, we swing into position to see a given point in the sky a little earlier every night.
Celestial objects are located on the sky by a coordinate system which includes a celestial longitude (right ascension) and latitude (declination). The right ascension of a celestial object is the same as the local mean sidereal time that the object passes an imaginary line drawn from the celestial pole to due south (180 degrees azimuth). Thus, if you know the local mean sidereal time (LMST) you also know what objects are in meridian transit at your location.