i (personally) need it for archiving the content.
so i get all the posts/thread without setting up a database + forum
you can open html everywhere, no need for (local) server. with no effort. for every system, for every user.
you can open html in 50 years, too.
you can archive only one or a few topics, not the whole forum.
you can give the archive to other ppl, and you don't have to clear the database (passwords, e-mail-adresses, privates messages, etc), because the html files only contain the public content!