Because the pressure in the plant doesn´t have reach the operation pressure, and is necessary more pressure for lubrication system in turboexpander.
Another reason is avoid the possibility of hydrates formation and reach the RPM of operation in turboexpander and the stabilization in demethanizer column.
You want the plant chilled down and fairly stable before putting the expander on. The expander needs to have clean dry and hydrocarbon free seal gas available at start up also.
