Captain Bob-
She is not floodable. What she does is hosts small craft (particularly SOC craft and mine warfare craft that are launched via cranes from the lower decks). On the upper deck she hosts SOC and Mine Warfare Helicopters. She also has berthing capabilities for these resources and allows them to operate much closer to the battle area than shore bases. As for the waste of $, I guess that is how you view it.

It looks like this ship has the capability to flood the hull like the Zhen Hua Chinese ships that deliver container cranes built in China to ports all over the world. But if so, then what's the purpose? The upper deck is flat, just like the lower deck. Tanks or heavy equipment could just as well be placed on the lower deck. This ship just looks like another colossal waste of U.S. taxpayer money.

She was built and entered into service as part of the Civilian manned Military Service Command. Hence the USNS - United States NAVAL Ship designation - similar to RFA - Royal Fleet Auxiliary.
After deployment to the Persian Gulf, for legal reasons she was commissioned as a full fledge US Navy warship. This was due to the primary combat role of the ship. In doing so she was on the few US Navy Warships that was commissioned outside of the US. And she is now known as "USS" - United States Ships and she is now an official US Navy Ship.
Interestingly she actually has a combination of civilian mariner and US Navy crew.
Confusing, yes, but it is what it is.

@ brian e cole & Bjorn Knudsen:

THIS IS IN FACT USS LEWIS B. PULLER. NOTE THE MISSING N.

http://www.navsource.org/archives/09/88/8803.htm

USNS Lewis B.Puller TESB3 - IMO 9656008

THIS IS IN FACT USNS LEWIS B. PULLER. NOTE THE N.

That is probably the strangest looking ship I've ever seen.

She is USS Lewis B. Puller ESB-3 now (Wiki):
https://en.wikipedia.org/wiki/USS_Lewis_B._Puller_(ESB-3)