Kids are notorious for asking "Why?" Each answer you provide leads to another "why." Each exchange leads closer to some central truth. With kids, that central truth usually ends up being, "Because I said so!" To designers, business stakeholders are also notorious. Not so much for asking why as they are for saying, "I think that should be blue," or, "This site needs more... sizzle!" In each case, the responses are easier to formulate if you have a guiding principle.
Ludwig Vies van der Rohe coined the phrase "Less is More." If you were standing next to him looking at one of his buildings, you might ask him, "Why aren't there any swirly things over the windows?" He would answer, confidently, "Because they are not necessary." The central guiding principle to his work is that the unnecessary is superfluous. Therefore, if it is not useful, it does not belong. The same is true of outstanding film directors. Every shot in the film is there to forward the plot of the movie. The plot itself serves to forward the central thesis of the movie.
A useful metaphor might be to think of design activities as legs of a journey. Let's say you're traveling somewhere and you've set the GPS to map out your route to your ultimate destination. Along the way, you and your fellow travelers become hungry. You're forced to abandon your route in search of food. You meander quite a way off course before you find food that is acceptable to your companions. When you are done, you need to reset the GPS device to get you from wherever you ended up to where you planned on going. Without an ultimate destination, you would continue meandering. Since you have a specific destination in mind, it's no problem for the GPS device to plot a new course to guide you ever closer to the end of your journey.
The same is true of the design process. Establishing a clear answer to the question "Why are we doing this?" will help you set and correct your course through each leg of the journey. In Enterprise Architecture, the course is set by decomposition of the corporate Mission. You take the Mission and ask what the company would look like if it accomplished its mission. That provides a Vision of what the company could be. You then further break that down into goals by asking, "What are the attributes of a company that fits our vision?" Once you've done that, you can outline measurable tasks that contribute to attaining those goals. We can call those Objectives. As a result, satisfying those objectives will help you attain your goals that complete your vision of the company you've set out to be. This provides a comprehensive set of answers to the corporate version of asking "Why?" This establishes traceability.
Establishing traceability in a design effort is invaluable in that it gives you a basis against which to measure the appropriateness of each design decision. That way, when a stakeholder says, "I think this should be blue," you have a logical chain of responses that ultimately ties that decision to the corporate Mission.
In order to make this less esoteric and more applicable, let's look at a specific case:
Let's say you've been tasked with designing a new website for a firm that provides professional services in the way of helping clients define their strategies and plan for the work necessary to deliver on those strategies (just to be fractally self-similar). When you ask the client why their redesigning their website, you get the obvious answer that it should sell more work. Digging deeper, you find that the corporate mission is to compete with the likes of much larger competitors. When you think about a firm that can do that, you realize that it needs to have a much higher level of brand recognition and be thought of as a trusted advisor. The goals, therefore, would need to be higher profile clientele, a deeper level of trust, and more recognizable brand. The measurable results would then be the number of clients that meet certain criteria, the number of touchpoints within that organization, and the number of people in a targeted demographic that recognize you and know what you do.
If you take that to heart, it means developing deeper relationships with people all along that awareness chain. You can then look at a particular shade of blue and decide whether or not it contributes to the measurable objectives of brand awareness. If that blue is not in your corporate palette, you can say that it's detracting from the users' ability to recognize your brand.
Ultimately, a clear view of the Mission, Vision, Goals, and Objectives of a design initiative or, more comprehensively, an organization, gives you an authoritative basis for Yea or Nay design decisions all along the way. It also allows you to correct course should you stray from the original path.