restrictions with nested simpleTypes.

I don't understand why the base attribute isn't good enough. Why would you
ever do this?

<xsd:simpleType name="fooType">
  <xsd:restriction>
    <xsd:simpleType>
      <xsd:restriction base="xsd:string">
        <!-- ... -->
      </xsd:restriction>
    </xsd:simpleType>
    <!-- ... -->
  </xsd:restriction>
</xsd:simpleType>

Thanks,
Jason.