Using familiar tools

HTTP, anyone? XML was designed as "SGML for the Web". Among other things, this means it was designed with HTTP transportin mind. This has led to developers using HTTP GET and POST for non-browser communications in protocols like XML-RPC and SOAP. Not everyone is entirely happy about it, but it's getting plenty of use.
Passing around objects, data XML's hierarchical structures match well to object structures. While it isn't always easy to map XML to objects in a useful way, it's generally extremely easy to map object structures to XML.
Text processing Fundamentally, XML is text. It's not always easy to process with regular expressions, but text-based tools can be very useful for creating and processing XML.
Parts is parts It's not hard to repurpose tools which were designed for the Web or designed to handle text messaging for use with XML.