Often, one is using the same string multiple times in a single application, such as a validation feedback message “Required field” or a label for a button “Confirm selection". This leads to a lot of duplication, which can be quite inconvenient (especially when you are supporting a lot of languages). Therefore, it would be nice if you could reuse your strings.
This same paradigm is also used when developing e.g., Android apps, where you have a strings.xml file (see also: https://developer.android.com/guide/topics/resources/localization)
Note: In a way, you could compare this to a string constant, but instead of just one value, the constant will have different values for different languages