Identity Transform

hello there,
why does the standard identity transform use copy instead of copy-of?
ie:
 <xsl:template match="/ | @* | node()">
   <xsl:copy>
     <xsl:apply-templates select="@* | node()"/>
   </xsl:copy>
 </xsl:template>

instead of just
 <xsl:template match="/">
   <xsl:copy-pf select="."/>
 </xsl:template>