The iPhone comes with either 8Gb or 16Gb of memory; why shouldn't you be able to use this memory as a giant USB stick to store your personal files / videos / mp3?
The answer is of course, that you should, but that Apple wants to control as much of this as possible. This is why there is no USB port on the iPhone, and no adaptor from the iPhone port to a standard USB cord. No -- the only way to get information in and out of the iPhone is via the web itself, which means using such arcane technologies as WebDAV , which is how the "Files" app gets around this problem.
When you launch "Files", the app gives you a"WebDav" URL. Note this down on a piece of paper and open up your Finder. Now, select "Go" and click "Connect to Server" and enter the URL exactly as you wrote it down. Through the magic of WebDav you will be able to drag and drop into and out of your iPhone onto your mac desktop.
If you can figure this out (and similar arcane instructions exist for different versions of Windows) you are well on your way to using your iPhone as a Storage device. Personally, given the poor reliability of the phone at this stage, I would rather store my files in the cloud. But -- yes, it does work -- at least on a mac.