WordPress is what is known as a CMS, Content Management System, i.e. it is a template for websites. There is quite a lot of CMS'es out there, where WordPress is the most popular. In 2020, roughly 35 % of the World's websites were using WordPress, which was equivalent to almost 62 % of the CMS market.
Why use WordPress?
WordPress has a lot of appealing properties. Being free is obviously important, but it is also a highly flexible system, so it can be used for a lot of other purposes than blogging, e.g. for webshops. What you also quickly learn to appreciate is that it is also a very stable system, and and does not inherently make the websites slow. If you are looking for help, there is also always help to be found on various forums and sites like this one.
Obviously you can also argue why you shouldn't use WordPress. It is hard to claim originality in the site's visual expression, when using a theme for the website used by millions of other users. WordPress is also a black box, i.e. you really don't know what is going on in the underlying code. Since WordPress is so popular, it is also always under attack from hackers and spammers. If you take a look at your log file, you will notice attempts at reading WordPress files on your site, regardless of whether you have WordPress installed or not.
Contrary to common belief, WordPress is not the solution for everything. You can do a lot with WordPress, and it is a good solution for a lot of jobs, but it does have its limitations. So, the recommendation from here, is to use WordPress if it is the best solution for the job at hand, not because it is the only system some mediocre web developer knows how to use, or because it is the most popular CMS at the moment.