I think the #1 reason to use WordPress is the plugins. Plugins are add ons that you can add to your website.
For example if I took one of my old hand coded html websites and wanted to add a contact form, it would require me to create of find a code for sending email, likely split into multiple files and then code it into the website. With WordPress I install my choice of a ton of contact form plugins and have it up and running in a fraction of the time it would take me to create one.
There are currently over 50,000 FREE plugins on WordPress.org. Not to mention free ones hosted elsewhere like GitHub, and large libraries of cheap plugins out there as well.
This means that rather then needing a programmer for the smallest function on a website, you install something someone else created and gives away for free and are done in minutes!
Of course nothing in life is that simple… There are times when plugins don’t work right, you can’t find what you want, etc… But these thousands of plugins do give a head start to any project.