Architects know design. Builders know how to build. They are two very different skills. I know this because I worked for sixteen years at a design-build firm and participated on both ends of that equation.
The architect is the originator, the guide, the person who knows the big picture of a building project. He (or she) focuses on the ultimate goal: to create a home or office that functions well; is beautiful; and can be built economically and efficiently.
The builder is the person who knows how assemble the details. He orders materials; hires subcontractors; puts the pieces go together in the right place at the right time; and figures out how much this construction is going to cost.
Both of skill sets are needed in order to build a home; but it's unusual for one person, or one firm, to have both. And you can't put the pieces together until you know what the pieces are and how they relate to each other.
That's why, when you're thinking of building or remodeling, it's best to start with an architect. He'll help you figure out the big picture so you can proceed to the details.