You should register a URL when it will be used in several places throughout your subsite. Then, if the URL were to change, you could simply edit the registered URL and it would be updated in all locations.
To check your registered URL or look for other registered URLs, run a Report. Filter for Registered URLs.
To update the Registered URL, run the Report and select Metadata under the Actions Column (first icon).
Click Standard Properties
Enter the new URL and save.