Think of your SEO consultant as an architect and your web development company as a general contractor. A general contractor can build you a beautiful home, but they need to have the perfect plans to work from. A good SEO consultant can put together the plans and requirements for your site and hand them over to your web development team before the build.
SEO is built into your navigation text, file names, graphic names and content. The navigational structure is also a major component of SEO. Unless you work with more than one person, it’s likely they are not an expert in both web development and search techniques because it’s too difficult to stay abreast of the changes in both fields.
If you are building a website to make money, then it’s worth your while to budget for the type of website that can work for you. Invest in both an experienced designer and an experienced optimization expert. The results will pay for themselves.