Webpacker in Rails 6 - Why do some default JS libraries have .start() methods?

When creating a new Rails 6 app, packs/application.js automatically includes the following:

require("@rails/ujs").start()
require("turbolinks").start()
require("@rails/activestorage").start()
require("channels")

Why do the top 3 libraries have start methods that need to be manually called?