Today: you find out last
You're managing five, ten, fifteen client sites — and the one time something breaks, you find out from an annoyed email, not from your own tools. Now you're apologizing for something you didn't even know happened, quietly wondering if this is the incident that costs you the retainer.
Juggling monitoring across a pile of tools that were never built for managing someone else's site — on top of everything you're already doing for your clients — turns "keeping them online" into its own second job.