The fact that duels were indeed illegal and, as Elinor states:

Elinor sighed over the fancied necessity of this; but to a man and a soldier, she presumed not to censure it.

What was JA's intent in including this little factoid in her novel?

What was she trying to illustrate about Col Brandon, I wonder?

I had always felt that, despite Marianne's and Willoughby's teasing, Brandon was in fact a very romantic figure. I believe he "felt" as deeply and as strongly as Marianne. It's just that, as a man of a certain age and position, it wouldn't suit him to openly demonstrate those deep feelings in the way Marianne might.

And I think that this duel just adds evidence to my own feeling about Brandon. He has a tragic past with a deep love, a cruel father, seeing his love suffer a terrible fate, saddled with a child and then seeing that child dishonoured and then falling for a young girl who doesn't return his love and having to fight a duel with her lover.... Wow -- Col Brandon could easily be the stuff of a regency romance novel.