At a certain point, everything you could ever want is available, so better things on land is pretty marginal. However, I agree, it seems unnecessary, but so does everything else they do. They literally have more money than they can spend, so why not I guess? Even if they don't use it, they can brag about how big their yacht is to the other billionaires, and I imagine some poorer people would be pretty enticed by it, if that's something they care about.