Why ActionController::Base.class_eval for includes?

Rails
#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