A few years back during a meeting I gave feedback on a proposed website design. I spoke as a potential user and was attempting to point out some possible pitfalls. The President of a company I was consulting for asked "Is this your personal opinion as a user, or as an advocate of the user?". Not an easy question as the lines are so fuzzy. Clearly this question is at the core of web design decision making.
One of the major rules of website design is to NOT build for yourself. There is a saying that "the lawyer who represents himself has a fool for a client". In my line of work, the saying should go "the designer who builds a website for himself has an expert as a client". And it is just as foolish.
When you design a website to satisfy your own needs as a user, your expertise will constantly pull you in a potentially dangerous direction.
You are engaged. The product is your job. You feel passionately about it (or are at least paid to be) and your willingness to read about the product, consider its merits, and reach conclusions are a given. Unfortunately you would be lucky if your customers are one tenth as engaged. Not because they don't care, but because they don't know.
You know the product. Exactly what it is and (maybe more importantly) what it isn't. Your customers know neither. And worse, many of them think they know what the product is and somehow have it wrong. As a designer you have precious little time to help clarify this.
You understand the process. You know what needs to happen and when. Moreover, you know what is difficult to successfully accomplish and what isn't. Your users aren't insiders like you. They don't know your operations. Your difficulties and triumphs are completely invisible to them. They will likely assume some things are easy that are in fact complex. They won't give them a second thought, meanwhile your success might depend on it.
You look at it every day. This causes a bias that work for you or against you. You know where to look and what to ignore. You may miss that your design is focusing the user on the wrong spot. You also might think your design is stale and "yesterday" - but perhaps that's because you spent all of yesterday staring at it.
Nothing can replace user feedback. The point is that the site is built for them - because without them you don't have a product.