SEO-friendly URLs can:
Describe your content
Help you avoid duplicate content
Avoid having your pages trigger spam filters
Make your pages easier to share, remember, and type into a browser
Help your Web visitors navigate your site
Increase search engine ranking for sought after keywords
The way that a search engine robot reads your URL can be different then how a human reads it. While you are structuring your website it is very important to create a URL structure that will please your mechanical visitors as well as your human ones. URLs that are too long, have underscores, capital letters, arbitrary numbers or even just look a little weird can cause problems in a couple of different ways.