The Molène Archipelago is the bigger reason the island matters: a protected scatter of reefs and islets inside the Iroise Marine Natural Park, with lighthouses, seabirds, seaweed beds, and a good chance of seeing seals from the boat.
If you can, choose a crossing or excursion that gives time around the archipelago rather than only a simple out-and-back ferry. The island is lovely, but the surrounding water is the real landscape.