Two reasons.
First — most of the businesses I audit don't actually need a full retainer. They need one or two specific things fixed. The Snapshot tells them which things, and they can decide whether to fix those things themselves or hire someone to do it. Either is fine. The cost of the audit to me is 45 minutes of focused work, and the goodwill it builds is worth far more than the time.
Second — most agencies dress up an automated tool as "expertise" and use it to bully business owners into contracts they don't need. That's the opposite of how I want to work. The honest audit is the lead magnet, the antidote, and the marketing — all in one.
If we end up working together after the audit, great. If we don't, you've still got a clear picture of where you stand. That's the deal.