Here's an example: It could be that a very young woman marries, immediately has children, never worked and elects to be a stay-at-home mom. After the children are raised, she then decides to go to work. She will first have to apply for a Social Security number if she hasn't had one up until that time. Then, depending upon how much money she earned and if she has worked a sufficient number of quarters, she will be able to collect an amount that these numbers indicate when the time comes.
Of course, she is also able to apply to her husband's account. She will be eligible for some benefits depending on his salary.