They are aerodynamic devices use to present uniform subsonic flow to the turbine.
At higher speeds a shock wave formes between the pointy hub in the middle and the outer case to slow the airflow in a uniform way to the turbine.
The cross section also changes normally expanding the cross section from the start of the intake to slow and cool the flow down before it goes into the turbine.
The main limit on how much power/thrust an turbine can make is the amount of heat it can add the the airstream in the engine without melting the metal its made from, the cooler the intake air the greater maximum thatrust/power that can be produced because you add more heat before reaching your temperature limit.
Many airframe desigers also put vortex generators on the outside of them to aid with high angle aerodynamics downstream of the pod.
Z
PS The bit on the front of the turbine is normally called an intake, or as on a 727 intake duct.