There are many reasons why you might want to update assets within an app at run time. This is often done within apps to provide announcements for new apps, or tweak small settings.
It can also be extremely helpful during development if you’ve got a larger app, and would like to make small changes.
There are of course limitations on what you can change in app if this is going to be an app on the app store (you can’t run code remotely).
Steffen Itterheim has created an example application that presents a webcam image in Cocos2D, and updates when that image changes demonstrating how to check whether the file has changed, and only downloading then.
You can find the example here.
If you are looking for an example on how to set up a server for easy variable tweaking while playing a game you can check out this previously mentioned tutorial demonstrating how to setup a server for easy app tweaking.