Like it or not, but tech people often have their way of doing things. Together we've drunk the same drink telling us that "this is the way of doing things", and we repeatedly create products where we incorporate this way of thinking, which go generations back (slight over-exaggeration).
This text is quite abstract and includes no concrete cases, but stay with me. I think everyone agrees on the fact that in interface and interaction design, there are some well-known and standard ways of solving problems. "This button should go in this view at the top right, and this kind of field should go here". "You should be able to input this kind of data here, and reach this view from that view". And so on.
But sometimes, it doesn't have to be like that. Sometimes, the most intuitive, pure and simple way of solving a problem comes from a single user.
This user is perhaps "untainted", i.e. not tech savvy/used to these kinds of interfaces. A blunt comparison would be a child: uncynical and who's seeing the world with youthful and optimistic eyes. This user may see things in our interface which really should be there, but isn't, since we've been drilled not to put it there since it's "crazy" and "isn't how it's been done".
This has happened to me not too many times. My parents and friends has sometimes outright said "Why can't I achieve <action> from here?". "Where can I see <feature>?". That was stuff I hadn't even imagined to put there! First, I had rejected it, since I thought it didn't make sense, but after some contemplation, I found it totally logical and obvious from a user's point of view.
Thus, I think testing on real users is vital in order for your product to be a product "of the people". The optimal thing (as I haven't done in this post) would be to scrap the term "user" and instead use "human" or "person" instead. They help you see things you couldn't see before. They are a medium between what your product is and what it should be, since we sometimes can't see through the barrier ourselves. Sketch below.
&lt; ------ | ------&gt; :D ^ User as medium
Look upon it as the story about Narnia: we can't access Narnia when we're too old, only live it through our children. So do the crazy thing, and go where you wouldn't go before. For fun and profit.