I've been a web developer for about ten years now.
I built SiteLetter because the gap in this category is sharp: most uptime and developer-focused monitoring tools were built for SREs running production infrastructure, then had a "reports" tab bolted on later. In those tools the agency was a workaround. SiteLetter is the version where the agency is the customer.
SiteLetter started as a way to check whether my wife's site was up. Then I wanted Lighthouse on it. Then visual change detection. Then a weekly email so I'd stop refreshing the dashboard. The product spiraled out of that. I still run it on her site every day, and that's the first place a real bug surfaces.