Most important reasons to have an SEO-friendly website structure
 

It makes navigation easier and allows users to find what they are looking for faster.
Improves website ranking because it is easier to crawl and index.
The bounce rate will be lower and the site will be well seen by search engines like Google: users stay on it longer, since it is useful and easy to use.
Increases the chances that the user will become a regular visitor.
Positively impacts site crawlability: search engine robots will find and rank more easily and your site will be more likely to be indexed.