Advantages of using key()?

Hello all,

What are the advantages of using xsl:key and key()?

For instance, instead of using this:

<xsl:key name="test" match="/root/child" use="@attrib"/>
.
.
.
<xsl:for-each select="key('test', 'special')">
<xsl:apply-templates/>
</xsl:for-each>

why not simply use this:

<xsl:for-each select="/root/child[@attrib='special']">
<xsl:apply-templates/>
</xsl:for-each>

Thanks,

Erik