Excerpt from a snapshot by Rick Jelliffe:
These standards are useful for several reasons:
1) They provide very detailed, public and usable information about the information exchanged by Microsoft Office products (Word, Excel, Powerpoint) including information that was discovered and disclosed during the process. For example, the exact conversion factors used in Excel for fluid measures. Public information needs to be free, and no tied up in mysterious or encumbered formats.
2) It will be the format used for the next generation of Office software. Trivial changes from the ECMA 376 specification (which is, excluding any minor SNAFUs, the native format of Office 2007) that were required by the ISO review process (changes I regard as completely bizarre) means that Office 2007 does not currently generate conformant IS29500 in a couple of trivial areas (e.g. "yes" must be "true" or "1".)
See the full post for more reasons and analysis.