First, the commercial services. And, just to ward off this question right away, no, you cannot access files or databases on a commercial service via a normal Internet account. You can access some of the commercial services over the Internet instead of over a modem, but this still requires you to have an account on that service.
Commercial services offer two main advantages over finding a real Internet site.
But, you pay for that easier access, usually with the connect-time fee for the commercial service. Second, the commercial services find it easier to offer commercial-quality information, because they can charge users to access that information and then pay the information provider. Hence you find, for example, full-text databases of computer magazines on CompuServe, but you pay extra for any searches in those databases, with the revenue going to the magazine publishers. In the last year or so, all the commercial services have added Internet email gateways, which means that you can use these services to send and receive Internet email. In the last few months, they've started to add extra Internet services as well.