[JET]

Actually the exhaust gas isn't releasing any energy by expanding. It will cool and the velocity will go down(newton's gas law, PV=NRT). The reason for the high pressure before the turbine is the restriction that the turbine wheel/housing places on the exhaust. This restriction causes the increase in temps and decrease in velocity.

The gas cooling and slowing after the turbine helps to slow the exhaust and allow it to travel through the exhaust more efficiently. The other effect of the low pressure on the other side of the turbine is to help suck the exhaust gasses past the turbine wheel, because of the pressure differential.