XPath is a very nice language for querying over XML. Most places pitch it as a "declarative" syntax, but as I am quite skeptical of "declarative" as a concept, you can also look at the vast majority of the XPath standard as a way to imperatively drive a multicursor over an XML document, diving in out and out nodes and extracting bits of text and such, without having to write the equivalent code in your language to do so, which will be inevitably quite a bit more verbose. When you need it, it's really useful.

In my very opinionated opinion, XPath is about 99% of the value of XSLT, and XSLT itself is a misfire. Embedding an XML language in XML, rather than being an amazing value proposition, is actually a huge and really annoying mistake, in much the same way and for much the same reason as anyone who has spent much time around shell scripting has found trying to embed shell strings in shell strings (and, if the situation is particularly dire, another third or fourth level of such nesting) is quite unpleasant. Imagine trying to deal with bash, except you have to first quote all the command lines as bash strings like you're using bash -c, all the time. I think "XPath + your favorite language" has all the power of XSLT and, generally, better ergonomics and comprehensibility. Once you've got the selection of nodes in hand, a general-purpose programming language is a better way to deal with their contents then what XSLT provides. Hence why it has always languished.