Imagine, that you have monitoring, everything is setup great and your metrics show green. But your customers can't purchase your product in your production or registration doesn't work.
This is why you'd want to set up something that would monitor that your product works in production from end-user's perspective.
Why is it a challenge?
There is an inherit challenge with functional monitoring: test stability. You don't want to be woken up in the middle of the night because a test flaked out.
How it should work?
The ideal way how you want your functional monitoring to run is:
1) To have a continuously running smoke test suite that would only notify you if there is an issue. This way if your customers can't buy a product or register you'd be immediately notified about it.
2) Get your notifications on all the right channels like Slack, Email, SMS, Phone.
3) Your tests are actually testing important functionality that if not working will have tangible business impact on your business.