We are all familiar with URLs: the string uniquely identifying the requested document. However, we don’t always consider they are more than that: URLs are user facing and should be considered important UI elements.
They should be clean, understandable, semantic, hierarchical and not excessively long.
And that’s why I have a (tiny) chapter dedicated on URL Design in the web courses I teach.
It’s also the reason why I claim API first instead of Mobile/Content First: thinking about an API forces you to structure & prioritize your data/content. When done right, structured data bears nice URLs — it’s a healthy side-effect.