What is the point of a namespaced root node in XML and where would such a namespace be defined?

I'm currently looking into a SPARQL ontology definition, which is in XML - something I am quite hazy on.

I'm familiar with the concept of namespacing. i.e. an XML node like this:

<rdf:Description rdf:about="...">
    <rdf:type rdf:resource="..."/>
    <rdf:type rdf:resource="..."/>
    <rdfs:domain rdf:resource="..."/>
    <rdfs:range rdf:resource="..."/>
</rdf:Description>

is referencing the rdf and rdfs namespaces, which are defined in the XML document root node:

xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"

And I understand then that this indicates to any XML parsing application that the node then is effectively:

<Description about="...">
    <type resource="..."/>
    <type resource="..."/>
    <domain resource="..."/>
    <range resource="..."/>
</Description>

With the tags (Description, type, domain, and range) as well as the tag-attributes (about and resource) unique in the context of parsing rdf XML. Which I guess the parser needs rules for - but these tags would never get confused with tags of the same name but defined for a different purpose. i.e. xmlns="http://finanacial-instruments.com/xmlns", where tags of the same name are used in different contexts (please let me know if this does not make sense).

The question

In this particular XML document that I am looking at, the root node is itself namespaced:

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF
    xmlns="http://meta.icos-cp.eu/ontologies/cpmeta/"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema#"
    xmlns:owl="http://www.w3.org/2002/07/owl#"
    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">

i.e. the root node is <rdf:RDF ...>. From my perspective this doesn't really matter since I already knew it was an RDF document. But what is the point of having a namespaced root node, and where is that namespace defined?

============== EDIT

Looking at the answer below, this link: http://www.jclark.com/xml/xmlns.htm helped me.

It includes the following:

<cars:part xmlns:cars="http://www.cars.com/xml"/>

maps to

<{http://www.cars.com/xml}part/>

I didn't realize that a namespace was also applicable to the tag in which the namespace is specified.