Free quotes and inspections prior to work commencing, time potentially spent waiting for jobs to come in in which customer support and taking calls are essentially a pure unpaid cost center for the business, taking the time to answer customer questions wholly unrelated to the job potentially even after the job completed, a desire by customers to hear you say things like

> hey, I saw your valve on blah was loose when it shouldn’t be, since I was there and it was an easy fix, tightened it for you as part of the job, no charge.

And there are likely more, I’m not a plumber.

If you are looking for it, you’ll find many cases of “free work is used as a tool to distinguish oneself, delight, and lubricate business”, and plumbing is no exception.