This has been a concept solidly supported by good, solid research since at least the late 70's. Most people who are not depressed are inaccurately optimistic. For example, ask college students what their future lives will be like and they're all going to find the perfect spouse, the perfect job, have the perfect kids, and be rich and happy. Sure, some will, but most will fall short of their expectations in at least one of those categories. People who are depressed are more likely to recognize that everyone doesn't get a bouquet of happy endings.

There's a lot of chicken-and-egg in this, I think. Generally, though, I'd say there are people who are optimists, pessimists, and realists, along a continuum. Manic and depressive states necessarily skew our views of reality more so than they already are. The question is whether our baseline perception tends to be more biased toward optimism (about ourselves, likely outcomes, others' intent, etc.) and if so why. Is there an evolutionary benefit to being a little unrealistic in our assessment of our own abilities? It strikes me initially as odd, because taking risks might work against survival — but I also see where it might aid in survival, giving us the drive and determination to try again and again until we succeed at something that helps us in some important way. A pure realist might assess a situation as having a slim chance, perhaps 1% likelihood of success, and would rightly determine that it's not worth wasting their time and energy on such a long shot; but for the optimist, who doesn't care that the odds are stacked against, the likelihood of success goes up just because they're more inclined to keep trying until they succeed. The realist isn't wrong, but the fact that optimists exist increases the likelihood of successes. It also seems likely to result in a lot of failure, so individually the realist may be at an advantage to be such — but the group may benefit from having some optimists as well. And pessimists have been shown to be important to projects because they're looking out for the pitfalls and safeguarding against worst-case scenarios. I'm a bit off the original topic, but I think depression does lead to some realism and some pessimism, but that's not necessarily a bad thing within certain moderate parameters. Ultimately, I think we need optimists for motivation who don't get discouraged easily (e.g., project leaders), pessimists who look for the negative and work to improve in the areas that are more likely to fail (e.g., quality control specialists), and realists who assess risk and determine what's worth pursuing given existing resources (e.g., upper-tier managers). I put it in terms of business projects as an example, but I think it could apply to multiple facets of life.