Let me rephrase things a little differently. On the assumption that you put the content into Drafts, what is it that Drafts does not do for you that BBEdit does do for you?
To give some additional context on the above, the following is based on my own views (/personal, rough, definitions):
Text editors allow you to quickly edit text in files (a wide variety of files) and the more advanced text editors give you tools to manipulate that text. Those advanced text editors also often give you ways to work in batch with such files and perhaps even build out to wider tool chains to build yourself a development environment for coding.
Word processors don’t allow you to manipulate the text as much, but they do tend to allow you to structure and format the layout. They aren’t meant to support coding, but they are there to help you produce short to medium form writing. Longer form you would probably be better off with a more specialised writing tool like Scrivener.
My personal view of Drafts is as a text processor. It isn’t a text editor, and it certainly isn’t a word processor. It is something that isn’t suited to working with files, or for building a coding environment. But it is suited to short and medium form writing (again I would say pulling together your memoirs is going to be easier using Scrivener or the like). As a ‘text processor’, what Drafts provides is quick access and an incredible expanse of tools for manipulation of the text. This could be internally, or through passing off to other services (local or online). In that sense it shares similarities with a developer tool chain, but is probably more suited to the sort of work carried out for writing, productivity, and knowledge management related tasks.
BBEdit to me is good for where you are working with files. Web pages, coding, sorting out files of data, editing text files. Drafts is good for capturing thoughts and information, building it out through writing, and then either storing it away, or passing it on elsewhere. For example, I don’t use BBEdit any more, but instead I use Sublime Text (I work cross-platform). I create a lot of my initial web site content in Drafts, but I rarely publish from Drafts (though I can). I typically push the initial content out to a file in my local site repository on my Mac, and then edit in Sublime Text. This is because at that point I’m then working with a file, and I can test all my site links etc. out on my locally served site before publishing it. Round tripping back to Drafts at that point gives me no benefit, but I have actions that can help me populate the initial draft version of a page pretty well, so there is benefit at the start. Once it is in a file, Sublime Text gives me quick access to all of the files in my site and I can work across them more efficiently with that, at that point.