Ok, I’ll try to explain as easy as possible with this example:
As you can see on the screenshot I have two objects that change their positions and rotate. It is a looped animation (the last keyframes are the same as the first one). Now I want to loop those properties (position and rotation) by copying those keyframes multiple times. But I want to make a shared copy so whenever I make any change in those first keyframes, those copies will change as well.
I could use a loopOut expression but the problem is I would need to put it in every property and in my real project I have much more objects. So now I would have tons of loopOut expressions and in case I want to make a change in the expression I would need to go to each one and make a change, lot of work. So the solution would be to have one expression linked to all those properties.
I know I could precomp this and loopOut the precomp but that would also not work because in my real project there are also objects that changes independently during the whole composition, so looping just one small part of the comp would not work.