Beyond version compatibility, IDML is a "cleaner" file. It is an XML-based format that removes the "bloat" often found in standard INDD files. Designers often export to IDML and then re-save as INDD just to fix buggy or sluggish files.
IDML is InDesign’s interchange format. It is a package of XML files that describes every element of your document—text styles, images, layers, and page geometry. Because it is not a binary format like INDD, IDML can be opened by any version of InDesign from CS4 to the present day. online converter indd to idml full