This is really useful if you aren’t sure if upgrading one package will mess up other programs, which happens occasionally with things like PHP and mail server libraries.
When you download packages, Ubuntu caches them in case it needs to refer to them further.
Take, for instance, Ubuntu Tweak, a program that lets you change some hidden or otherwise difficult-to-change settings for your system. If you add the repository instead of downloading and installing just the package, the system will notify you of updates and automatically keep it up-to-date for you.
You can manually add and change repositories by editing APT’s sources file: That will update the package lists from all repositories in one go. Now that you’ve added your software repo and updated your package list, and found the package name you need, you can install it.
This will download and install all of the packages listed.