From what I understand the subscription plugin allows you to charge registered users a subscription price for access to the "internal" (registered users only) content of your website.
You might have material on your website for everybody to see (without logging in), and material which requires logging in. Here the subscription module comes into play... You can only log in if your subscription is paid....
This is not simply a "money making" device, but a sensible protection against spammers and hackers who otherwise might register to your site for free to do all kind of "stuff" your site is not intended for (including advertising for their products etc.)...
There are components already which handle this kind of taks, but they are complicated and not as well integrated as one would wish.... So we are looking forward to CB's very own plugin.