Tell me if you ever ran into this scenario: you have an app (say, a bank app) and that app has a file that you need (for example, a bank statement in PDF).
You want to download that file, but the app doesn't offer any easy way to do so, the max it offers is the option to share the file, meaning you end up having to send the file to a messaging app or to yourself via e-mail to even be able to access it.

This happens because some apps decide to not provide a Save option, this is usually due to the devs not wanting to implement the logic for saving a file themselves, however they usually provide a Share functionality to instead take advantage of the built-in system share system.