Let's say you have module1 that is linked to module2. You decide module1 is the wrong type of module. Perhaps it is watching Mailchimp subscribers rather than getting a specific subscriber.
You delete a module1 and replace it with a new one, module3 that does what you want.
Unfortunately, module2 is looking for data from module1 with a module ID of 1.
All of your data mapping will have a "1." prefix.
If you want module2 to work with the new module3, it looks like the only way to do that is to manually change all the mapping.
You would think that you could just change the module ID of module3 to "1" but I don't see how you can do that.