"I'll get a reminder from the certificate issuer, so I don't need another service to monitor them."
"I've got auto-renew set up"
"I've got Certbot set up , so I don't need to worry about it".
But... are you'll sure get the reminder email? Will it end up in your spam folder? Was it registered by you, or someone who's now left the organisation?
And what happens if your credit card expires and auto-renew fails? Or Certbot has a problem?
You'll probably be fine, but if something does go wrong your users and customers may not be able to access your site, or will get worrying warnings from their web browser, and that may impact your site's reputation, revenue and even search engine ranking.
The bottom line is, it never hurts to double-check.
SiteSentry will check your site's SSL Certificate every day and start warning you 14 days before it's due to expire, so you have plenty of time to renew it.