2. Just for Fun.
They are enjoyable toys. They can be utilized for leisurely travel and entertainment. They are bought by billionaires for the same reason they acquire private planes.
It is obvious that wealthy people today prefer to buy luxury yachts for cruising. By employing such yachts, they can experience a delightful voyage.
4. Status symbols.
The main justification is that the yachts are significant status symbols. According to rumors, the Emir of Abu Dhabi owned the Azzam, while Steve Jobs, Paul Allen, Roman Abramovich, and Steve Jobs all owned Venus (complete with its own submarine and missile defense system).