Generally, the pie crust is lined with tin foil or parchment paper, then filled with dried peas, lentils, beans or other pulses, or with ceramic "baking beans", so that it will keep its shape when baking. Metal or ceramic pie weights are also used. After the pie crust is done, the pulses are replaced with the proper filling. Blind-baking is necessary if the pie filling can not be baked as long as the crust requires, or if the filling of the pie would make the crust too soggy if added immediately.
Blind-baking is essential if the filling is not intended to be baked, for example a chilled filling.
Blind-baking also helps to form a nice pastry case for the filling as it has already been partially formed from blind-baking.