The author of a book is never the same person who edits it. This is because the author becomes closer to that finished product than anyone else; therefore, they are likely to overlook mistakes while editing. For the same reason, they may have a somewhat emotional attachment to that finished product, where a constructive outside view is needed to bring it into perspective.
All of these same issues can be applied to the development of a digital platform. And, perhaps most important, we know that users can differ tremendously in terms of mindset, behavior, digital capabilities, and so on. This means that there is a likely chance that the audience of your digital platform is going to differ tremendously from the person designing it.