Why ActionController::Base.class_eval for includes?

rubyonrails-talk
1

Just out of curiosity, why does action_controller.rb include classes with class_eval instead of directly? I.e., why

ActionController::Base.class_eval do   include ActionController::Flash   include ActionController::Filters   ... end

instead of just

class ActionController::Base   include ActionController::Flash   include ActionController::Filters   ... end