I think its to reduce the isolated nacelle/pylon drag coefficient by reducing the boundary layer drag, keeping the airflow attached over the pylon, and to weaken any shock interference caused by the nacelle.
Pod mounted engines also produce lift when they are placed at an angle of attack, this is normally express as a function of the wing angle of attach, nacelle incidence angle, and the nacelle upwash or downwash angle.
The design of engine nacelles, pods, and mounts is a tricky aerodynamic challenge to make sure the engine get clean undisturbed air in all normal attitudes, and to reduce as much as possible the interference drag of the installation.
Pods mounted in front of the wing have an upwash angle (like A320), behind the wing a downwash (like B717), you can use the upwash to generate additional lift on the nacelle.
Also pods are normally designed so that the have a zero local sideslip, sometimes it is necessary to toe-in the nacelles like on a B747 to achieve this.
