[xsl] Using Key in the middle of Xpath

["Mailing Lists Mail daktapaal@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Guys,
where I consult, I was going through the XSLTs to check if they have
all been done correctly... I came across this Xpath which I thought
was a little unheard of,. I am thinking it is wrong usage, but want to
confirm with you guys.

Xpath :

<xsl:variable name = "foo" select = "//aaa/bbb/key('mykey','myval')/fooval"/>
where key is :

<xsl:key name = "mykey" select = "ccc/ddd" use = "@id"/>


The code has been working ( I wondered if it was by a toss ) .. But I
have never ever seen a key being used in the middle of the Xpath. For
me it does not make any sense of the use of key.

Comments?

Dak