As part of the design process, we gather our thoughts on how to achieve a delivery in a way that can be persisted. As a byproduct of carrying out the task, a form of reference material is built up that can prove useful in the future.
Note that the design will not be maintained after creation, and therefore does not indefinitely offer details of how the system will be - it is very possible that the system has been changed since our first design was carried out. However, it can offer a reference as to why we delivered the solution in the way we did - it exposes our thought processes at the time.
The intent of the designer can prove important at a later date, where it becomes important to ensure that any changes to the system do not have unintended consequences. This is not a reference you will refer to a lot, but it is something that is beneficial on occasion. Remember, this reference was generated as a byproduct of the process, not as the key deliverable. Its benefit is not great enough that we consider it sufficiently important to spend time doing it, but we get it for no extra cost after following the design process.