You're writing an article and you're confident about an assertion like the following: "Historians [or sociologists or ...] have shown X," but you can't dig up the references right away. So, you e-mail a colleague in whom you have confidence and ask, "As far as you know, it is true that historians, etc., have shown X," and your colleague e-mails back, "Yes, indeed, X." Then you cite the e-mail to support your assertion.
The example (with the first e-mail in the chain imagined but the second and the footnote not) is drawn from a recent article in a T-25 law review. My guess is that the footnote's there because the law review editors wanted support for the "Historians have shown" assertion, and the author had a very short deadline.
(In the same issue, there's an article with an early footnote fully identifying a recently published source, than a series of much later footnotes referring to the source in manuscript form -- pretty clearly someone got tired of updating the footnotes to reflect the recent publication; it' an exceptionally long article even by law review standards.)
The example (with the first e-mail in the chain imagined but the second and the footnote not) is drawn from a recent article in a T-25 law review. My guess is that the footnote's there because the law review editors wanted support for the "Historians have shown" assertion, and the author had a very short deadline.
(In the same issue, there's an article with an early footnote fully identifying a recently published source, than a series of much later footnotes referring to the source in manuscript form -- pretty clearly someone got tired of updating the footnotes to reflect the recent publication; it' an exceptionally long article even by law review standards.)