It's simple. The number of checks converts to consumption of application resources.
The lowest subscription plan includes a low number of checks and simple features (like only uptime monitoring).
The higher plans offer more checks and more advanced features that need even more resources (like page speed monitoring or transaction monitoring).
That way, if you need only simple monitoring for your small website you don't have to pay too much.
And if you're a web agency or a large e-commerce store, you still can find a plan that suits you.
Number of checks hasn't much to do with the features you need.
One might need 50 simple uptime checks for 50 small sites, but other would need to monitor 5 different features on 10 websites (like content, loading speed, forms).
For any website:
* content (to confirm if the page is displaing properly)
* proper functioning of web forms
* domain expiration
* SSL certificate expiration
* presence on blacklists
* search engine robots blockade.
For more complex websites (ecommerce, web apps):
* proper functioning of particular processes (check-out, registration, log in, etc.)