WordPress is used here because it allows you to control your information rather than have it trapped by some web company. It is free to use, but running a website come with costs.
So what is it exactly?
It is free software (Open Source) that lets people set up and maintain a website.
Open Source Software is where there is a legal agreement to share the programming code with the public. This allows people to use, study, share and modify the code.
While there are many other website software options. Some of which are easier and more flexible than WordPress. I use it due the fact that it is maintained by a large global community of of volunteers, paid staff and companies. Thus making it one of the largest Open Source projects in the world.
Plus, so long as you diligently back up your website, you always have control of your information. As you can easily move your website to another web company, so you are not tied to one company.
There are two WordPress projects that can be confusing:
WordPress.COM is the company that sells hosting, themes and other services.
WordPress.ORG is the project where the Open Source Software of WorpPress is maintained, updated and planned.
Due to the popularity of it, around 40% of all websites use it,
many individuals and companies make additional software to give it additional functionality. These are called plugins, they range from image galleries, to online security, credit card merchant, newsletters, etc. Many are free and the more premium ones are a one time purchase or subscription.