XML offers the tantalizing possibility of truly cross-platform, long- term data formats. It's long been the case that a document written on one platform is not necessarily readable on a different platform, or by a different program on the same platform, or even by a future or past version of the same program on the same platform. When the document can be read, there's no guarantee that all the information will come across. Much of the data from the original moon landings in the late 1960s and early 1970s is now effectively lost. Even if you can find a tape drive that can read the now obsolete tapes, nobody knows what format the data is stored in on the tapes!
XML is an incredibly simple, well-documented, straightforward data format. XML documents are text and can be read with any tool that can read a text file. Not just the data, but also the markup is text, and it's present right there in the XML file as tags. You don't have to wonder whether every eighth byte is random padding, guess whether a four-byte quantity is a two's complement integer or an IEEE 754 floating point number, or try to decipher which integer codes map to which formatting properties. You can read the tag names directly to find out exactly what's in the document. Similarly, since element boundaries are defined by tags, you aren't likely to be tripped up by unexpected line-ending conventions or the number of spaces that are mapped to a tab. All the important details about the structure of the document are explicit. You don't have to reverse-engineer the format or rely on incomplete and often unavailable documentation.
A few software vendors may want to lock in their users with undocumented, proprietary, binary file formats. However, in the long term, we're all better off if we can use the cleanly documented, well- understood , easy to parse, text-based formats that XML provides. XML lets documents and data be moved from one system to another with a reasonable hope that the receiving system will be able to make sense out of it. Furthermore, validation lets the receiving side check that it gets what it expects. Java promised portable code; XML delivers portable data. In many ways, XML is the most portable and flexible document format designed since the ASCII text file.