John Baden explains why periodic downturns are important for a healthy economy. It would be nice if market participants always made smart decisions, but they don't, and downturns are part of how resources are reallocated to those who can manage them well.
Every successful society has devised ways of separating incompetent or systematically unlucky people from the control of valuable resources. (That's why civilized nations provide children and legally incompetent individuals with guardians and trustees.) This is an essential process for all but the most wealthy of nations, e.g., those cursed by great oil wealth. (This windfall wealth situation is the national analogue of individuals winning the lottery; a harbinger of bad things that follow the lack of a need to husband resources.)
A society's economic success is increased if it has sure and quick ways to accomplish this separation, however painful to those who suffer losses. While there will be political pressures to buffer folks from the consequences of economic folly or bad luck, it is socially dangerous to do so. Reality checks should have force, so that those who fail to prudently manage resources will not keep control over them.
We all benefit -- directly or indirectly -- when our civilization's resources are managed by the most competent among us.