Think about the condition we currently call the "disorder" ADHD.
We can easily imagine how the behavioural tendencies ascribed to it would have been beneficial in hunter-gatherer times; i.e., it would be beneficial to the tribe for a certain number of people to be restless and explorative, rather than being conformist and content doing repetitive tasks. Same goes for bipolar; think about how many of our leading writers, inventors, musicians are diagnosed bipolar, and how useful it would always have been to have had a few people with that kind of creativity and propensity for bursts of novel productivity in the tribe.
And I can easily think of ways depression and anxiety are adaptive and supportive of survival, rather than not (for depression it's the ability to contemplate/reform a troubled life, and anxiety it's the avoidance of danger). Even schizophrenia in certain circumstances.
And of course when we invoke Occam's razor, it makes far more sense that these genes are there because they're beneficial, rather than being there due to a historical accident.
It can also mean that the normal state of the brain exists near a small number of other stable modes, and that a wide variety of small nudges would have similar effects. The article mentions that a study of schizophrenia found that it was characterized in part by extremely rare mutations, suggesting that the brain is at an unstable local maximum, vulnerable to degrading into schizophrenia if unbalanced in any direction. Schizophrenia may in a sense be a much hardier fallback mode of brain operation.