Often, the repositories that come pre-installed with Ubuntu will suffice for the majority of your server needs. Every now and then, though, you may need to install an additional repository in order to take advantage of software not normally provided by Ubuntu, or versions of packages newer than what you would normally have available. Adding additional repositories allows you to subscribe to additional sources of software and install packages from them the same as you would from any other sources.
Adding additional repositories should be considered a last resort, however. When you install an additional repository, you're effectively trusting the author of that repository with your company's server. Although I haven't ever seen this happen first hand, it's theoretically possible for authors of software to include backdoors or malware in software packages, and then make them available for others via a software repository. Therefore, you should only add repositories...