We like to believe quality speaks for itself. It does not.
Unfortunately, people judge the book by its cover. Always have.
You can have the best product, the finest work, the most useful idea. But present it carelessly, and it will be seen as careless. Present it with thought and detail, and people assume that same care exists underneath.
The cover does not change what is inside. But it shapes how the book is judged, at least until someone decides to open it.