Welcome to Oldgas.com
There is some lettering on the side of the fixture base. What does it say?
It looks similar in purpose, but not quite like the typical Revere brand island "mushroom" lamp that has an inverted bulb and cast aluminum harp.
Mushroom shade pole island lights were used at stations without a canopy from the 1930s to 1950s when they were gradually replaced by more efficient fluorescent fixtures.
Lots of collectors want island lights to enhance their outdoor gas station pump display or to use as a driveway light at their garage.
Value is always dependent on condition. Most of these style light shades have a porcelain finish. Yours looks to have been repainted at least twice. Chipping appears to go deep into the porcelain. If the paint could be chemically stripped off to reveal nice porcelain, it would be desirable, but that is probably not your situation. The top spotlight appears to have a broken base connection which could be challenging to repair because of the unique way it connects to the harp. Collectors who want a shade with nice porcelain and standard Revere top spotlight setup would pass on buying which would hurt your value. But since it is different from most island lights, some collectors would want to try to repaint and repair it just to have something different.
Here is a link to day & night photos of a station with island pole lamps, different from yours, but they show how they were used:http://www.oldgas.com/forum/ubbthreads.php?ubb=showflat&Number=748642#Post748642