In an ideal world, all documentation content would come in one format (and that format should be DITA). But let's face it, content produced in a company is diverse and comes in many forms and sizes.
So how can we single source everything? Can we integrate contributors who use formats like language-specific API documentation, HTML, MarkDown or even Excel spreadsheets or database tables in a DITA-based workflow? Could we convert everything to DITA on the fly? Could we use a magic glass to perceive various data sources as DITA?
We may try to convince everybody to produce DITA content but this may not be always possible. Instead of that we can accept these diverse data formats but look at them as different ways of encoding DITA. So if we put in place the right decoder we will get back our DITA content.