Question

Validation requires that I set it but why? I like to think the code in my xhtml document is doing something.

Answer 1

From the W3Schools:

the xmlns attribute specifies the xml namespace for a document.

This basically helps to avoid namespace conflicts between different xml documents, if for instance a developer mixes xml documents from different xml applications.

An example of this (also from the W3 website):

XML data to define an html table:

<table>
  <tr>
    <td>Apples</td>
    <td>Bananas</td>
  </tr>
</table>

XML data to define information about a coffee table

<table>
  <name>African Coffee Table</name>
  <width>80</width>
  <length>120</length>
</table>

There are two table elements here, which would produce a conflict. To fix this, you can add a namespace to signify which piece of information defines an html table and which comprises information about a coffee table:

<root>

<h:table xmlns:h="http://www.w3.org/TR/html4/">
  <h:tr>
    <h:td>Apples</h:td>
    <h:td>Bananas</h:td>
  </h:tr>
</h:table>

<f:table xmlns:f="http://www.w3schools.com/furniture">
  <f:name>African Coffee Table</f:name>
  <f:width>80</f:width>
  <f:length>120</f:length>
</f:table>