All the reasons given above and the fact it can make a "fishing village" into a production powerhouse in the early game. In many games I get at least one city (often in the arctic or on an island) that has say 2 good seafood tiles but no hammers apart from the 1 from the city tile and no useful land tiles for cottages either. Getting +8 food and 1 hammer with a size 3 city is great with slavery as soon as the granary and lighthouse are built. You could use the food surplus to run specialists (but the GP farm will outproduce any GPPs) so I usually turn it into a whipping / drafting city.