"For what purpose?". That would be your reaction too. Well, that was mine when I heard saw the lighthouse in the 15th arrondissement in Paris. The rocky cliffs and the sea are hours away from Paris. To stretch it a bit, the lighthouse might be for the Seine river. But the river is also far away from this location. So what is the purpose of this lighthouse?
The lighthouse announces the location of local fish market. When the local fish market was setup, the lighthouse was bought and transported to this location. In the olden day Paris, this would have been a good move. The lighthouse stands tall and visible from a distance. Anybody who want to buy fish just have to look up, find the lighthouse and walk towards it.
Yes, there is a fish market even now. Like a fish market, it smells of fishes. To be honest, I love the smell.