Validator.w3.org - why?

Can I get some opinions on the markup validator?

What is the point of it? I’ve never known the reason behind someone using it. Like it seems to find errors in code defaultly added in by Dreamweaver. I don’t understand it.

If your web sites display perfectly in all the most commonly used browsers, why should I care if the validator finds errors in the code?

Can someone please explain to me the practicality of it. I honestly don’t know. I’m not saying there isn’t any use for it. I’m actually asking because I don’t know.