It allowed for the 10+ tidal range. A crane lifted the crates to the fixed pier above. All the post-tie ups are fixed to the floating dock.
I took this photo with a long lens so the slag heap in the background is in fact, quite far off. This is the lobster pound on the Western end of Stonington Harbor on the Deer Isle Thoroughfare. Xmas was anchored off Crotch Island, the Spruce covered island in the far background, a 10 minute row away.