The process of upgrading your Island Sanctuary and increasing your Sanctuary Rank in Final Fantasy XIV is one that can take a variety of forms, including gathering materials, building facilities, or crafting Isleworks Handicrafts. While Island Clams aren't used to create any of the Island Sanctuary gathering tools, nor commonly used in the construction of impressive landmarks, they are used to make a variety of items in the Island Workshop, and keeping a healthy supply of clams can be critical to maximizing workshop output.
Beyond being a good source of Island EXP, the Island Workshop rewards players with Seafarer's Cowries, a currency used to purchase a variety of items and exclusive rewards. Shrewd players with an eye for gil may recognize this as an opportunity to not only quickly rank their sanctuary up, but earn some gil while they're at it, making Island Sanctuary attractive for casual players and would-be entrepreneurs alike.

Where to find Island Clams
Island Clams can be obtained by activating Gather mode and interacting with nodes labelled as Large Shells. No special tools are required to gather Island Clams, so players can begin building up their supply as soon as they unlock their own Island Sanctuary.

Conveniently, Island Clams are only found in one location off the coast of Pirate Bay in the northeast corner of the map, also indicated in the in-game Sanctuary Gathering Log. This gathering area is underwater, so players will have to dive in order to access the nodes here. If you're looking to farm up Island Clams, note that there are only eight Large Shell nodes here, requiring you to gather from some of the Seaweed Tangles as well if you want to respawn the Large Shells.