I have no idea why, but
I mean, it's not really about San Francisco; it's really about him. Yes, I know, he was sitting on a pier that ran into the San Francisco Bay when he wrote the key line, but the rest of the song was written by Steve Cropper--and he purposely wrote it about Otis. Evidently, Otis hated writing about his life, but Steve found it full of inspiration.
I guess the rhythm of the song does remind me of sitting at some of my favorite beaches along the coast--Pescadero Beach in particular. Or better still, of sitting on the pier of my great-aunt and great-uncle's place on Tomalas Bay, just north of San Francisco. They sold it in the mid-80s, when I was but a wee lass, but I still remember going fishing with my dad off that pier, and sitting on it to enjoy the sunset.
I guess its purpose is to have us reconnect with some vague memory...it does with me.