Why is XmlNamespaceManager necessary?

I've come up kinda dry as to why -- at least in the .Net Framework -- it is necessary to use an XmlNamespaceManager in order to handle namespaces (or the rather clunky and verbose [local-name()=... XPath predicate/function/whatever) when performing XPath queries. I do understand why namespaces are necessary or at least beneficial, but why is it so complex?

In order to query a simple XML Document (no namespaces)...

<?xml version="1.0" encoding="ISO-8859-1"?>
<rootNode>
   <nodeName>Some Text Here</nodeName>
</rootNode>

...one can use something like doc.SelectSingleNode("//nodeName") (which would match <nodeName>Some Text Here</nodeName>)

Mystery #1: My first annoyance -- If I understand correctly -- is that merely adding a namespace reference to the parent/root tag (whether used as part of a child node tag or not) like so:

<?xml version="1.0" encoding="ISO-8859-1"?>
<rootNode xmlns="http://example.com/xmlns/foo">
   <nodeName>Some Text Here</nodeName>
</rootNode>

...requires several extra lines of code to get the same result:

Dim nsmgr As New XmlNamespaceManager(doc.NameTable)
nsmgr.AddNamespace("ab", "http://example.com/xmlns/foo")
Dim desiredNode As XmlNode = doc.SelectSingleNode("//ab:nodeName", nsmgr)

...essentially dreaming up a non-existent prefix ("ab") to find a node that doesn't even use a prefix. How does this make sense? What is wrong (conceptually) with doc.SelectSingleNode("//nodeName")?

Mystery #2: So, say you've got an XML document that uses prefixes:

<?xml version="1.0" encoding="ISO-8859-1"?>
<rootNode xmlns:cde="http://example.com/xmlns/foo" xmlns:feg="http://example.com/xmlns/bar">
   <cde:nodeName>Some Text Here</cde:nodeName>
   <feg:nodeName>Some Other Value</feg:nodeName>
   <feg:otherName>Yet Another Value</feg:otherName>
</rootNode>

... If I understand correctly, you would have to add both namespaces to the XmlNamespaceManager, in order to make a query for a single node...

Dim nsmgr As New XmlNamespaceManager(doc.NameTable)
nsmgr.AddNamespace("cde", "http://example.com/xmlns/foo")
nsmgr.AddNamespace("feg", "http://example.com/xmlns/bar")
Dim desiredNode As XmlNode = doc.SelectSingleNode("//feg:nodeName", nsmgr)

... Why, in this case, do I need (conceptually) a namespace manager?

******REDACTED into comments below****

Edit Added: My revised and refined question is based upon the apparent redundancy of the XmlNamespaceManager in what I believe to be the majority of cases and the use of the namespace manager to specify a mapping of prefix to URI:

When the direct mapping of the namespace prefix ("cde") to the namespace URI ("http://example.com/xmlns/foo") is explicitly stated in the source document:

...<rootNode xmlns:cde="http://example.com/xmlns/foo"...

what is the conceptual need for a programmer to recreate that mapping before making a query?

The basic point (as pointed out by Kev, above), is that the namespace URI is the important part of the namespace, rather than the namespace prefix, the prefix is an "arbitrary convenience"

As for why you need a namespace manager, rather than there being some magic that works it out using the document, I can think of two reasons.

Reason 1

If it were permitted to only add namespace declarations to the documentElement, as in your examples, it would indeed be trivial for selectSingleNode to just use whatever is defined.

However, you can define namespace prefixes on any element in a document, and namespace prefixes are not uniquely bound to any given namespace in a document. Consider the following example

<w xmlns:a="mynamespace">
  <a:x>
    <y xmlns:a="myOthernamespace">
      <z xmlns="mynamespace">
      <b:z xmlns:b="mynamespace">
      <z xmlns="myOthernamespace">
      <b:z xmlns:b="myOthernamespace">
    </y>
  </a:x>
</w>

In this example, what would you want //z, //a:z and //b:z to return? How, without some kind of external namespace manager, would you express that?

Reason 2

It allows you to reuse the same XPath expression for any equivalent document, without needing to know anything about the namespace prefixes in use.

myXPathExpression = "//z:y"
doc1.selectSingleNode(myXPathExpression);
doc2.selectSingleNode(myXPathExpression);

doc1:

<x>
  <z:y xmlns:z="mynamespace" />
</x>

doc2:

<x xmlns"mynamespace">
  <y>
</x>

In order to achieve this latter goal without a namespace manager, you would have to inspect each document, building a custom XPath expression for each one.