Erm... don't really know for sure but my best guess is that they would perform a function similar to wing fences - ie. prevent separation of the airflow from the nacelle surface as it flows around and up the inside of the engine. Separation would create unwanted drag, and possibly unstable airflow over the slats/intake
Passengering with a window seat on cold, humid days I have noticed that condensation trails form above and behind the fin, indicating a pressure drop - I suspect this is a vortex forming behind the fin to 're-energise' the airflow and preventing laminar separation.
Or not?
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...proceeding below Decision Height with CAUTION...