Many years ago, publishing executives in the business press, which has become more commonly known as b-to-b, set up many business magazines with a controlled circulation. This simply means that the publishing company sought out readers for its magazines who would most likely be interested in the content.
These individuals were subsequently sent invitations or circulation forms to receive the company's magazines or maybe a singular magazine. If the recipient replied in the positive, they were then signed up for a free one-year subscription that needed to be renewed every year. The reason for this arrangement is simple. In order for the magazines to sell advertising to companies who wanted to send their sales message to a given audience, the publisher had to ensure that such an audience was assembled and be able to prove it to the advertiser.
Thus, to make sure that such an audience existed and was perpetuated, the publisher sent the magazine(s) to such recipients every month for free. Thus, an audience was created for the advertisers and a business model was created.
We still use this model.
One could make the argument, however, that the really worthwhile magazines should not need to worry about perpetuating an audience if their content was really valuable. There is a lot of merit to this notion.
However, most of us publishers are hesitant to rock the boat and ask our very interested reader/subscribers to pay for their subscriptions even though they probably would. Thus we have the controversy of controlled circulation vs. paid circulation.