During the course of submitting a form, users often need to trigger workflows, macros or send notifications. Although all of these can (and have) been achieved using Microsoft Flow and Azure Timer Functions, these would only be triggered when the user saves the form.
An example would be a user filling out a medical billing form. As soon as the persons identity number or insurance information is entered, the user may want to have that data immediately validated while they continue with the rest of the form. The user would click a "Verify Insurance" button which would trigger asynchronous JS that calls the insurance companies API, and returns the result while the user is still with the form.
If the user doesn't click the button in the form, then the data would still be validated using Microsoft Flow or similar.
Another example would be to escalate a record while the user is working on it. The user clicks a "Notify Manager" button which alerts the manager that a record needs attention. (naturally this would be tricky if the record is in create mode - but not impossible).