Why does unemployment matter to investors?
Unemployment should matter a lot to investors, since pretty much everything comes back down to how well the economy is performing. The companies that issue shares need customers, and without high employment, people can't afford to spend as they're accustomed, which can depress entire industries. For example, if money is tight, people will not be buying new cars, and that may restrict profits for automakers.
The other main reason unemployment really should matter to investors is that it can be a broader economic indicator. High unemployment points to poor economic performance, whether it's a cause or an effect (often both), which can then create a whole cycle of pain for investors. During these down cycles, stock prices may fall, or companies may go bankrupt, and these are both situations to be aware of.
Falling stock prices are less of a concern to long-term investors, since they do generally rebound, eventually, but bankrupt companies aren't good news for anyone. When times get tough, you may see investors selling their shares and buying bonds (or just keeping that cash on the sidelines), which further depresses stock prices.