[Evolution] What exactly does "preformatted" do?

["George N. Reeke" <reeke mail rockefeller edu>]

Can someone tell me (or give a link to) exactly what "preformatted"
in the composer dropdown does when composing new mail?  Documentation
seems to assume it is obvious; from the name it might have to do with
how inserted existing text is handled; googling tells me it turns off
col 72 line breaks--good, sometimes I want that.  Before blindly using
it and making a fool of myself, I would just like to know all the
things it does.
Thanks,
George Reeke