Use cases
Same failure. Different thing you stand to lose.
For agencies
Every client site checked, alerts before the client calls, and reports that go out with your branding on them.
For freelancers
Your client should not be your monitoring tool. Free for one site, nothing installed on their hosting.
For marketing teams
Broken tracking wastes budget and broken forms lose leads. No developer needed to set it up.
Campaign launch
You have already paid for the click. Catch a broken landing page before the budget goes with it.
E-commerce
Your checkout is up. Whether anyone can actually buy is a different question.
Migrations
Baseline the site before you move it, then prove the new one works. Both hosts answer — that is the problem.
WordPress
Your site updated overnight. Did the menu, the form or the theme survive it?
WooCommerce
The storefront looks perfect and the payment fields never appeared.
Shopify
Shopify keeps the platform up. Nobody is watching your theme, your apps or your cart.
Netlify
The build passed and the preview looked fine. Neither of them visited production.