Make your most loyal customers into owners. They are often your most passionate evangelists.

Investors have a financial interest in helping your business succeed. For instance, if you own a brewery and your customer is also your investor, she'll likely drink more beer there, and bring her friends more often. 

Need help recruiting an engineer?  Looking for an intro to a key VP at a big company?  Want feedback on your product from an industry insider? A crowd of investors can help increase your "luck surface area".