It's a fair question. You've got a Facebook page, you post regularly, customers leave reviews, people message you directly. It feels like a website. In some ways, it acts like one. So why bother paying for something separate?
The short answer: Facebook is borrowed space. A website is yours. That distinction matters more than most people realise.
Facebook can take it all away tomorrow
This isn't scaremongering. It happens constantly. Accounts get restricted, pages get unpublished, ad accounts get flagged. Sometimes for a clear reason, sometimes for absolutely no reason at all. When it does happen, you have no real recourse. You can appeal, but Facebook's support is notoriously slow and unhelpful.
If your entire online presence lives on Facebook and that page disappears, so does your ability to be found. No website means no backup. Years of posts, reviews, and followers. Gone.
Your Facebook page belongs to Facebook. They set the rules, they can change the rules, and they can remove you from the platform at any time.