Here are some reasons I have come across for why WordPress is so popular in the 12 years that I have used it.
Easy to Use
WordPress is easy to update. The dashboard is very intuitive with an area for Posts, Pages and Other Custom Post Types. Adding content often only requires filling in a title, some details and a photo, selecting a category and then clicking publish. Custom post types are basically things like posts that you can publish for example events. You would add the title, description, date, time, location, category, photo and publish. A common example of custom post types are portfolio posts or projects. These are especially good for people who want to showcase their work from graphic designers through to builders. Other useful custom post types include FAQs, products, galleries, slideshows, services, locations, courses, team members, reviews / testimonials, service areas etc. A lot of themes are built in a way that make them easier for non-experts to add and change these post types.
With over 1/3 of websites using WordPress, there are a lot of people at various levels searching for answers. Most of the time someone else will have had the same issue or question about WordPress as you and a simple Google Search will often yield the answer, whether in a blog post or YouTube video. YouTube has loads of content about WordPress. On top of that you can also pay a small fee for a course on Udemy if you want. There is also WP Beginner.
There is a very large community of developers that are constantly putting out more updates to improve security and performance. If you come across a security problem, the solution will be along soon or possibly already been dealt with an update may all be what is required. Using a very bespoke setup for a project is like going into the desert on your own. Using WordPress is like going on an adventure with a lot of backup on call – coastguard, rangers, police, ambulances etc.
The fact that WordPress is used so much does mean that if for some reason your WordPress designer or developer stops operating or falls out with you, then you can usually find someone else to keep the project going.
A WordPress plugin is basically a package of code that adds some functionality to the website. So for example, I use the Gravity Forms plugin for making simple and complicated Contact Forms. It is also common to use plugins for page speed (making your WordPress website fast), ecommerce (WooCommerce is the best known), backups (sending backups of your website to remote storage), security (preventing and finding malware), booking systems and more. A lot of functionality can be added using free plugins. Simply go into the WordPress dashboard and go to plugins and add new and then just search. You can also purchase plugins. I have purchased plugins for all sorts of different functionalities.
WordPress has a lot of themes that can be used to give different designs and functionality on the website. Like plugins there are lots of free and paid plugins. I have used both. For charities on a shoestring budget I have used some free themes as a starting point to make smart simple modern websites. For customers willing to pay more and looking for a more polished result I usually start with a paid theme and then build from there. Using a theme saves hundreds of hours in development and correspondingly many thousands of pounds. Yet the result is pretty much the same.