It is a really excellent question.
This is why a national research strategy for the brain will work. It's because all of these disorders are related to one another. Some of the briefs submitted by organizations in our coalition have presented some data on this.
Having a basic, fundamental understanding of how the system works will let us better fix it when something goes wrong. We have been finding this more recently in the last few decades. For example, Alzheimer's disease has a large component of inflammation or it causes the immune system to react, perhaps in a bad way. It turns out that other diseases that we know have involved immune components, such as multiple sclerosis, might draw on these same mechanisms at the base. Having an understanding of the brain and how it develops through all of the different ages is going to contribute knowledge that will bring therapies and cures to other diseases as well.