As the world shifts to computing on the cloud , or what my non tech friends call, lots of servers connected to the internet, APIs become increasingly the lingua franca or working language to communicate to and fro from these clouds. Storage in the cloud has been of the earliest use cases and one which has increasingly made sense to create secure backups at a much cheaper rate.
So, you're trying to share some files with someone who only has access to an old version of Word. Or maybe that Microsoft-hating friend of yours who only uses WordPerfect. Perhaps you want that old-school ebook to be an epub, rather than .lit. For any of these problems, there are desktop apps that work, of course. However, sometimes you want to be able to do that within a program. If so, Doxument and the beautiful Doxument API are what you need.