What does the url in XML file mean

Question

I find this URL (or a similar one) always on HTML files, XML, XSD... Like "http://www.w3.org/2001/XMLSchema" or "http://www.w3.org/2001/XMLSchema-instance" I always wonder what those URLs means. Even offline the XML or HTML document works without changes. What's the benefits on links to those URLs?? Thanks

Answer 1

It's the XML Schema.

An XML schema provides a view of the document type at a relatively high level of abstraction.

Answer 2

Those URLs do not necessarily point to any website/server. They are a convenient naming mechanism. The idea is since every company will have a unique website, using that as their namespace will avoid clashes. Hence better interoperability. Hence the custom.

Namespaces in XML 1.0 Specification