When we create development work for Prospective clients we are generally ask develop a website that is a Search Engine Optimized .That things bring me into a long-winded explanation on how developing a website to be search engine friendly is not the same thing as optimizing that website for search performance.
The SEO provider or consultant should be involved in the website development process, though those two roles are completely different. The developer designs and creates the front and back end of the website. A good developer can do that job well, but mostly they do it to function for the user, not perform for the search engines. The SEO should be involved to ensure that what the developer produces is as search engine friendly as possible.
Most web design and development companies don't know the first thing about SEO or creating SE friendly websites. And most who claim to are liars. Sorry, but that's just the way it is. Design/Developments are two completely different things but they are closely related to Search Engine Optimized.
The process of SEO requires hours of additional research and a skill set that usually is not included as a part of the site development contract. There are a few development firms that also specialize in SEO and Marketing and these firms can develop and engage in long-term optimization services. But be aware that any short-term website development contract that claims to include SEO simply will not provide adequate or successful SEO marketing results.