While launching and scaling a website/online store/app is indeed possible using website builders and Content Management Systems (CMS) like open source WordPress (using library of existing themes and plugins) that should be sufficient for small businesses, yet as an owner of a small business, you will want more customization at some point. It is then when expertise in web development comes to action.
Here is the objective of Front End Web Developer Nanodegree program by Udacity:
While front end web development takes care of visualization/user experience in terms of way the website appears, back end or full stack web developer will be in a position to add new functionalities (plugins in the world of WordPress).
Here is the objective of Full Stack Web Developer Nanodegree program by Udacity:
The goal of the Full Stack Web Developer Nanodegree program is to equip learners with the unique skills they need to build database-backed APIs and web applications. A graduate of this program will be able to design and build databases for software applications, create and deploy database-backed web APIs, and secure and manage user authentication and access control for an application backend. Students will also learn how to deploy a Flask-based web application to the cloud using Docker and Kubernetes.
The above two courses by Udacity are not limited to a particular technology/software. If you use WordPress, you can still benefit from the above two courses while implementing your learning in WordPress environment that uses PHP language. Udacity does not have any course specific to WordPress. Udemy has a large number of courses on WordPress.
Front end WordPress development
Back end/full stack WordPress development