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, they'll likely drink more beer there and bring their friends more often.
Do you need any help recruiting an engineer? Are you looking for an intro to a critical VP at a big company? Want feedback on your product from an industry insider? A community of investors can help increase your "luck surface area."