Is BlogML Created Only for Migration Between Blogging Engines?

No, the answer is negative! Although BlogML has been used for this purpose in most scenarios and this can be the most common application of BlogML but BlogML is a more general thing and that is an XML format for blog content.

This means that every application for such a format can be an application for BlogML. For instance, it can be used to back up and restore the content of a blog which is applied by some bloggers.