If you're like me and are building up settlements but have one as the main settlement you're working on, then you might want to open up supply lines to that main settlement. Beats from having to go around and scavenge for junk for each place, as that can take a while. A supply line allows you to use the resources you have between the two settlements, without having to transfer any of it to the settlement you're currently working on.

After further development, you can indeed loop your settlements, meaning if you wanted to, you can share any resource, from any settlement that you have connected via supply lines.