In many cases, paying for a service you could do yourself is more about having a throat to choke in case something goes wrong. I'm sure I could figure out cron/crontab if I took the time to learn about it but that's not my core skillset so paying $5/month is actually rather cheap for me (especially when the cost is passed along to a client who is less skilled at these things than I am).
What makes cron-job.org not an alternative for us, is that this they monitor by crawling a URL that you provide. This is limiting and doesn't serve our use case for backend system monitoring.
By contrast, Cronhub gives us a public endpoint that we must ping by a certain interval to say that our service ran in the interval.
We also use their slack integration to show any monitoring failures in our #fires channel!
It's a monitoring service that "just works" and we don't have to maintain it, and if things go wrong we can reach out to someone since we are paying customers.
I’m sure plenty of people thought the same thing about statuspage, yet they seem to be doing fine. People like easy and centralized. I think it’s a great idea, and there’s room for value-adds later down the line.