OK some of the peole who have answered this have got it wrong...
it was not (and is not) unique - far better stone circles survive. However, it is one of the best known remnants of Druidic (and possibly pre-Druidic) religion in the world.
Certainly, it is alligned so that during the summer solstice, the Sun rises alligned with certain stones - in a similar way to how other ancient buildings are alligned with the Sun or stars at certain dates (the Pyramids, for example). It has also been suggested that it charts the rising of the Moon and the setting of the Sun on certain dates too.
So possibley it was used as a giant calender (although religious in use) - the Druids were able to interpret astronomical data through ceremony, amnd allow the farmers to know when to sow seed, when to best hravest etc etc.
As the Druidic religion was destroyed by the Romans, we do not really know what or why Stonehenge really was. But it does seem to have been developed continuously for thousands of years, and so was probably an important site for the monolithic culture. The monolithic culture existed in Europe from modern Spain, up through France, in Britain, North Germany, and Scandinavia, but not much is known of it - it was destroyed by the Romans and other invaders, leaving only its relics such as Stonehenge and some burial mounds.
The monolithic culture fascinates people as it was how humans lived thousands of years ago in what is essentially Western Europe, but we know less about it than we do of the Ancient Egyptians, Greeks, Persians, Incas etc.