"Though hidden from the sea, the castle controls access to Loch Shiel" says Wikipedia. Which is a sensible thing for a castle to do back in the olden times.
Plus it's a tidal island - you can just walk across. Don't even need a drawbridge (although I guess that makes defending attacks from inland a bit tricky.)
That said, there's an Apple TV video of a castle on an island where my first thought was "why does this exist?"
https://vimeo.com/657386068