Be the one who calls them
There is an enormous difference in a client relationship between "your site is down, why did you not know" and "we spotted an issue on your site twenty minutes ago and it is already fixed". The technical work is identical. The perceived competence is not.
This alone tends to justify the cost of monitoring across a client portfolio, before considering anything else.
Certificates are the recurring emergency
Agency work involves domains registered by clients years ago, with renewal notices going to an email address nobody checks, on hosting the client set up before they hired you.
Automatic certificate tracking on every HTTPS monitor turns this from a recurring surprise into a scheduled task. A thirty-day warning is enough time to chase down whoever controls the DNS.
Uptime reports as a deliverable
Retainer renewals go more smoothly with concrete evidence attached. A quarterly summary showing uptime percentage, incidents detected, and response times gives the client something tangible for the money.
It also reframes the conversation. Instead of justifying hours, you are reporting outcomes.