I do some consultancy work for the company, Aquarium Architecture, that installs these types of big tanks for rich folk. There's more than you may realise. With the footballers it's often a case of one-upmanship; my fish tank is bigger/better than yours etc. etc.
That's good for the company and its employees, and the footballers etc. get a nice feature in their home. The tanks are maintained regularly so the livestock is well-cared for.
Interestingly planted tanks are becoming more popular so we can expect some equally as impressive large planted aquascapes. Unfortuntely for us perhaps, not all clients are as interested in making their installations public knowledge.