It has to do with building a proper foundation—getting the bones built before synthesizing the more superficial layers. If the bones are mismatched and out of place, the system of the body may not work properly. The same goes for building a highly functional and usable website.
In How Website Structure and Information Architecture Should Mirror Your Business Goals, Trond Lyngbo talks about how we must deeply understand the psyche of our target market and then attempt to solve their problems and insecurities in the form of SEO and visual design. Research and design thinking strategies must be used in order to grasp a bigger concept of who the user is and what he/she wants (or is looking for). All of this is considered behind-the-scenes work that must be done in order to guide visual design.
It may have once been that SEO consultants and Visual Designers butted heads, but a good balance of the two strategies is key in building any Interface. One does come before the other, however, and it would be detrimental to a business if visual design came first, before “building the bones.”
The Information Architecture is the structure of the website. Continuing with the body metaphor, an individual with a healthy metabolism (and proper diet and exercise), will appear different than an individual with an abnormal metabolism with the same diet and exercise. The inside of the system affects what we see on the outside.
IA affects what appears on screen, just the same. It is the inner workings of the the system that shapes the look and feel of the visual design. With great visual design and the support of IA and SEO, websites can do what they were meant to do more accurately and more fluidly, which is to provide information!