It's there "just in case". Where it might be useful, for example, is if you publish something to an online service and for some reason it's deleted (ie not through LR). Alternatively, you may find a publish service doesn't automatically mark images to republish when you think it should have done. For instance, the publish service may not have been coded to treat a change to the caption or the collection name as a trigger for republishing. Republish just lets you force the service to output the files again. In the rare occasions it's needed, it is good to have.
