Why? It's good for making sauces that need body, but without any overpowering flavor from a brown veal stock or jus. Ducasse uses it a lot for simmering fish heads and making gelées. I guess you could use a chicken stock made with wings and feet too, but this is much more gelatinous.
They are potatoes. Agree about the odd numbers, though in this instance 6 fit the plate better.
The lobster gets blanched then sliced and sautéed in its shell in olive oil. The shell protects the meat from overlooking and adds great flavor. Also looks cool on the plate.