Schematics all refer to it as a "sensor dish" so who knows. Could be used for super-sneaky passive directional sensing in low observability conditions like a dense nebula or finding the blind spots in a sensor network, or fine navigational sensing through an asteroid field, a dense nebula, or even a black hole cluster -- a gaping maw, if you will.
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My bad ...
Who says it's stationary? In the first one, I believe it's shown in several different positions.
Another fun fact: the escape pod Rey uses in Episode 8 was present in the 1996 toy, but they changed the glass cover a bit in the movie.