What is the importance of the middle class?

Why is income inequality a problem? There are more really rich now, and there are more poor, and this is a trend that has been moving steadily since 1970. So the middle class is shrinking in terms of numbers.

How is this a problem for the middle class? Does a shrinking middle class cause a cultural shift? Does it change attitudes? Tastes? Styles? What does it do to the zeitgeist? What’s so important about the middle class?

Obviously this is a problem for those sinking into poverty, but I’m wondering if there is any problem for the middle class—if having fewer members makes a difference to them.