There are WordPress Developers and "WordPress Developers".
Confused? Let me explain.
When I first got started in web development over 3 years ago, I thought the only way to develop a WordPress site was to use the WordPress editor itself.
I had no idea that I could leverage the powerful backend tools that WordPress provides out of the box, all while putting together my own custom design and functionality.
WordPress Developers create Custom Themes, Custom Post Types, and Custom Plugins so that "WordPress Developers" can make modern and highly functional websites.
Custom Themes
Have you ever wondered who's actually making the themes you see when you're searching for new themes?
Those are made by WordPress Developers. By using PHP and built in WordPress functions, developers can make modular code that can be shared and customized by the end user, or the "WordPress Developer".
Custom Themes can be sold or can be saved in a codebase to be easily reused and repurposed for your next client.
Custom Post Types
Custom Post Types may be the most powerful feature of WordPress.
They tap into the way a typical blog post works, but allow you to completely customize the way the link to the post shows as well as how the post itself shows up. The WordPress framework has already set up your entire database to work this way, all you have to do is follow a few simple rules.
Leveraging Custom Post Types elevates the "Wordpress Developer" experience and also allows for greater code modularity and reusability.
Custom Plug-ins
Plug-ins are the "WordPress Developer's" best friend.
In a nutshell, Plug-ins are bits of code that can be injected into your existing WordPress project to add more functionality. Much like Custom Themes and Custom Post Types, making Custom Plug-ins gives you ultimate modularity with your code.
WordPress Developers can sell their Custom Plug-ins or use them to drastically cut down their own development time.