I have often wondered what difference it makes to earn, say, £3 million a year rather than £2 million since both seem to be far more than I could ever spend. But I suppose 80% of the world's population could be wondering what on earth the average UK adult earning say £20000 can possibly feel they need more.
Often it's relative and it's about recognition. If one footballer happily earns £100000 a week and then finds a similar player earns £150000 he (or more likely his agent) may well suddenly feel he is underpaid. If all your friends are multi millionaires it's understandable to want to show you can afford a similar lifestyle.
Also people want to put money aside for the future and their kids.
I know though that even if I won £5 million in the lottery I'd still shop around for deals and wouldn't see a need for a flashy car.