Why does plaster of Paris have 1/2 molecule of water? Why does 2 CaSO4 molecules share one molecule of water?

Plaster of paris is Calcium sulphate hemihydrate . [CaSO4.1/2 H2O]
Please Note that it is not possible to have a half a molecule of water.
The Formula actual means that two molecules ( or two formula units ) of CaSO4 share one moecule of water so that the effective water of crystallisation for one CaSO4 unit comes to half molecule of water.
The formula of plaster of paris can be written as 2CaSO4.H2O
2(CaSO4.2H2O) -----> 2CaSO4.H20 + 3 H2O
on heating