You should see Page Rules as a way to create exceptions to the settings you apply to your website. Say for instance you domain is example.com.
Crypto > Always Use HTTPS
All pages from example.com, www.example.com, api.example.com, m.example.com etc will be redirected to HTTPS for visitors requesting http.
Now let’s say you need for some reason to have api.example.com NOT using https. Then you should turn Always Use HTTPS off on the dashboard, and create rules to match the other parts of your website for which you want HTTPS on. For example.
api.example.com > some settings
example.com Always use HTTPS
So that for the API section you’d not have Always Use HTTPS, for everything else, you would.
You should ask yourself this: do I want all pages from my website on HTTPS? If the answer is yes, then you should set this at the dashboard. If the answer is no, then you have to create the appropriate page rules.