publicurl configuration option was added to S3 special remote to facilitate public access to the files deposited to S3 via their HTTP "frontend". For many remotes (e.g. rsync, directory, etc) it might happen that the remote location directory is also served by a regular HTTP server. So it sounds very reasonable to enable regular HTTP(/https) access to those files publicly by providing those special remotes with publicurl setting so annex could simply try to access those files via http. In particular relevant for the special remotes with exporttree=true.