Some people think that no matter what URL structure you have, search engines will be able to make complete sense of it and be able to index the site. While this is true, to an extent, they then use this idea to say that there is no such thing as SEO-friendly URL structure. Clearly, they missed part of the point of using “friendly” URL structure.
While making sure a site is crawlable and loads fast enough are important to an SEO, because they require both to do their jobs, SEO is more focused on getting people to the site, ranking that site as high as possible, and getting a solid ROI.
Web developers are the other people working on a site that are concerned with URL structure, but they are usually those thinking any URL structure will work. They have different focuses than SEOs and as long as the URL is indexable, they have been successful in that aspect. In other words, what web developers consider to be an alright URL structure may not work for the SEO working on the site.
There are a few simple rules to making a SEO-friendly URL structure. Firstly, the URLs should be straightforward, with absolutely as few redirects as possible. Making everything organized helps keep the site structure more cleaner and keep visitors from getting confused. You should also make the URLs meaningful. Keywords, or just in general some form of language, is better than seemingly nonsensical numbers and punctuation. Most importantly for SEOs, it is important to know which URLs are most important, and which need to be hidden from search engines.
Alesia Krush explains SEO-friendly URL Structure at Search Engine Journal, but most importantly she shows how you can achieve it on your site in just five easy steps.