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."