As the other comment says, for better diffability, but also it's good to store the document in a "neutral" format and then render for the eventual display - MS Word, PDF, HTML.
If you store Markdown in a database it would be easier to full text index and update as well EG "We've changed our Product Name to SuperWidget from MidWidget. We need to do a global find and replace on all our docs."
For the writers markdown diffability is bad since it mixes style and content
Strange to call markdown neutral given how it's tied to HTML, and that you could just as well convert your "non-neutral" rich format to the same targets
How is Markdown _Super_<span style="color:blue">Widget</span> easier to globally find&replace vs. anything else that you create an index for in a database?
Yes, those two would work better for the writers since then they'd be able to WYSIWYG-edit individual documents in other editors while having the potentially better style/content diffability/batch renaming etc in this one (and that would also help avoid the common mistake of having two separate views just because the underlying format is plaintexty)
Although personally I hope for a modern rich-text CRDT-based format that would also enable collaboration, but don't know of any specific one in that area
If you store Markdown in a database it would be easier to full text index and update as well EG "We've changed our Product Name to SuperWidget from MidWidget. We need to do a global find and replace on all our docs."