capistrano, :db role, what's it for?

As far as I can tell, the capistrano :db role is used only to run migrations. (Thus, in most cases it probably shouldn't actually be the server that runs your database. Why would you have a ruby/rails stack there (or allow ssh logins there)? it's just whatever server you want to actually execute the rails migrations). And only the server identified as db role with :primary => true is used to run m