What is the encryption for?

I have recently setup my nextcloud server with external access, and have been testing it with a bunch of PCs. I have now installed the Nextcloud iOS app, and I am curious what this encryption is.

What does it encrypt?
Why is it needed on iOS and not a PC?

The encryption file is available and compatible only for ios device.

Sorry, but it still don’t understand.

On the welcome page of the Nextcloud iOS app it says that it will keep ALL my private data from “prying eyes” by using AES-256 encryption. I read that as when I start using the iOS app, all my files on Nextcloud will be encrypted. Seems very strange.

What files are encrypted? Only the ones uploaded from the iOS app? How is this coordinated with the desktop app and webpage where I do not use encryption?

I just don’t understand what the encryption is doing. I can go anywhere in the world and log onto my cloud using a random computer with username and password, but when I use the iOS app things get encrypted?