Ask HN: What's the purpose of investors and the stock market?

What with Marissa Meyer getting Yahoo's CEO post, and FB's stock dropping like a stone, I've been seeing plenty of half-formed blog posts about the troubled relationships between tech companies and investors. As a person who's new to tech, it's hard to figure out what you guys traditionally think about "the markets."

So HN, what do you think the purpose of investors actually is?