Put them both together, and you have a method of tranfering multiple chunks fo data in one shot, where extraction for execution can take place on one (where that may make use of the rest).
When you request data from the likes of a POP3 (mail) server, your client sends formatted commands, designed to extract data. The returned information is also formatted.
XML allow for unlimited changes in the actual types of data, and/or addition of data, while maintining the existing data, so it can still be extracted.