Equatorial mounts are designed to compensate for the rotation of Earth, by matching Earth’s axis of rotation with its own rotation parallel to that axis (Right Ascension), as well as a perpendicular axis to that of Earth (Declination). This type of mount is used for astronomical telescopes, and long exposure astrophotography (should the mount have computerized object tracking). A counterweight is typically used for balancing the OTA (optical tube assembly) to reduce much of the stress on the mount’s motor drive gears.
Polar alignment is necessary to properly track the an object in the sky. Polar alignment allows you to find objects using the standard Declination and Right Ascension coordinates used by astronomers and astrophotographers. This is essential for computerized GoTo equatorial mounts, which depend on this system.
Once you have found an object by whatever method, an accurate polar alignment keeps the object in view while observing – only the Right Ascension slow motion control is needed to track the object from one end of the sky to the other; no further adjustment of Declination will be required. This is necessary for longer observing and astrophotography sessions.