If any web site pages are created before SEO is performed, those pages will likely be using the wrong web page URL / filename, diminishing the prospects for that page or delivering off topic traffic. If the wrong HTML coding techiques are used, search engine robots may not be able to traverse the site. They can also be misinformed as to what the page is really about.
If any of these situations occur, the page will have to be torn down and at least partially redesigned when SEO is actually performed on the page.
First, an SEO specialist will to talk to you and get a general idea of what you want the site to consist of, what its goals are, how many pages, what the pages are generally about, etc. Then the SEO specialist's job is to go out and do keyword research, competitive website research, run some probability calculations (if not on paper, in their head), and then develop the following list:
Next, content text is developed, as well as graphics for the site. The SEO specialist then has to make sure that the web designer uses the right web page programming techniques so SEO is not adversely impacted. All of the information is then as a blueprint to build the site. Graphically and layout wise the web designer has reasonable freedom.
SEO is about getting the search engines to understand about a page and to attempt to control what they think the page is about. Strategically SEO is also about helping to decide what to write about in the first place, based on what people are already searching for.
If SEO is done first, everything else will follow.