We've both noticed that for nontechnical owners of small businesses that have a heavy software component, that owner is completely at the mercy of their lead developer. If that person leaves or "gets hit by a bus" their company might just die, especially since they don't really know what a Github is or how to log into anything. My friend is such a nontechnical person and hired someone to build his SaaS app. He has felt this risk himself but we wonder if he's just particularly perceptive.
The service would be, for a retainer fee per month a developer sits in on your planning calls, looks over all code committed to your repo, and over time documents how everything works and stores that documentation in a system outside of the company (maybe an external wiki or something). Then if your lead developer leaves, we'd help you find a new one and then train them on the codebase using the documentation created.