So for me, it doesn't make sense that you call illustration "interpretive" (yes!) but then say the message/content PRECEDES it - how can it? It is not a message or content until it takes the final form as an illustration (data and text might really actually precede... there's a nuance for us; I don't think we can treat all four terms as synonymous).
And so.... this leads me to qualify the idea that illustration is occupied with reportage, explication, didacticism, etc. If all illustration is indeed interpretive, then all illustration is creative and theoretical and even personal, and therefore it cannot help but be _expressive_ as well as didactic. And it is the expressive aspect that drives stylistic and aesthetic qualities, which I would suggest illustration is ALSO occupied with, equally; qualities that, while they help the didacticism, actually operate in excess of the message too.
I willingly concede that a list of primary purposes cannot hope to include all attributes of an individual work, let alone a creative enterprise. But why should it be expected to? The challenge before the field ia lack of analytical discourse. My aim here is to articulate some basic analytical language to frame discussion. I don't think it's debatable that illustration is a responsive activity. A text, a unit of content, a required subject is presented to the illustrator with an expectation that a tailored interpretive thing will come back. That this process can be collaborative, that tracking it can be complex, that the resulting product may be in some way "personal" or "expressive" does not change the function of the image: communication in the service of the larger work. I would argue that function must be separated from style. The insistence upon illustration being personal and expressive has led us into the blind alley of "Hey, this is art, too!" Illustration may be artlike, but it is not, in the philosophical sense, Art. The distinction may be unwelcome, but better to be clear than not.
The last point will require more explication. More soon. Thanks for the comment and engagement. Keep it coming.