Yachts are a popular way for extraordinarily rich people to rub how much richer they are than just regularly rich people into their faces. Costing tens or hundreds of millions of dollars, they're essentially mansions that float. But what do you do when you want to rub yacht owners' noses in your even greater wealth? You turn that yacht into a submarine, naturally.
The U-010 is basically a yacht-submarine combo. When surfaced, it's got a huge deck for relaxing and taking in the sun. But it can then submerge underwater by switching from diesel to electric propulsion. The interior is, as one would expect, pretty over-the-top, but for what you'd pay for this, I guess that's what you want.