The container is just a good base for beginners. I agree it could be clearer, maybe some pre-defined sizes when you drag that container in with 940 being the default.
But, yep, exactly as you said. Drag a container in a set a max-width. The most standard is 1140px the bootstrap grid. But rules are meant to be broken, so you can play around with what works for the site you’re building. If you download the Visual Inspector extension for chrome you can click and view what other sites are using. These days people use various sizes.