WordPress manual

You don't pay a single cent for monthly hosting, domain registrations, or SSL certificates as long as you are building.

No one can accidentally peek at a site that is not yet finished. No nosy competitors and no search engines indexing a half-empty page.

Want to keep start-up costs low

and have no desire for slow or complicated systems and unnecessary plugins.

You can test endlessly, make mistakes, delete everything, and start over. You won't break anything on the real internet.

by wanting to understand how their site works themselves and not wanting to be dependent on a developer for minor adjustments.