The Grand Mosque of Saint-Louis adds an important counterpoint to the island's colonial balconies, old administrative buildings, and Catholic cathedral. Its presence makes clear that Saint-Louis is not just a French colonial relic; it is also a Senegalese city with a long Muslim urban life.
Non-Muslim visitors should be respectful and usually appreciate the building from outside unless invited in. It fits naturally into a walk across the island, especially if you are paying attention to how different communities have shaped the same narrow streets.