Think of it like when you change the aspect ratio of your image on your camera, lets say you change the aspect ratio to take pictures in square format. However, what is happening is the camera is performing an in camera crop of the image data. Now think of shooting RAW plus JPG in square format, you get the jpg square image but the raw is the full sensor data and so includes bits of the image that the crop removed. This means you can go back later and change your crop should you wish to.
This is a generalisation of that same thing here. despite you framing a certain way, it stores the entire image with a crop applied for viewing that you can change later. For those damn, I should have cropped that lamppost in/out... type of thing.
Are there any downsides to turning it on?
I tried reading about this feature, but it actually sounds rather involved what all iOS does to your photo.
Being old-school, I generally prefer maintaining control and NOT letting software tinker with my photos. (e.g. Something I read implied that iOS somehow updates your photos later on and/or threw away some data - I didnt follow what was beings aid...)
Good photographers compose BEFORE they shoot, and so they are less likely to need all of this after-the-fact trickery.
And, personally, I "bracket" the hell out of things, so I almost always have the composition that I want anyways...
What I am gettinga t, is if I leave this turned off, will I lose anything as long as I am careful to copmose what I want the first time?