The basic situation is that people work for about 40 years, during which they earn more money than they spend, and then they retire and don’t work for about 30 years, during which they spend more money than they earn. That’s a very broad statement, you could quibble with the numbers, and it is certainly not true of everyone: Some people retire early, some retire late, some die young, some live a long time, etc. But as a rough statistical average, most people get some decades of saving in the workforce and then some decades of spending in retirement.