Without Checker (the usual approach)
You hear about an SSL or site outage from your client
Lost traffic, rankings and client trust while a technical failure or a 500 error goes unnoticed for days.
Hours of manual error hunting
Hours of manually hunting for broken links (404/500), lost meta tags and deleted pages after a new site release.
No idea why your rankings changed
When you make changes to the site, it's unclear which of them drove results up or down.