Because there is so much free porn available on the internet (e.g. TGPs), some users have long decided to consume only free porn, and thus spurn pressure to become paying customers. However, paysites are often one of the main originators of new content, they're one way an administrator can pay for the bandwidth costs, and they're one of the most conspicuous ways in the industry to try to turn a profit. Theoretically, the more revenue a site takes in, the higher quality content it can produce.
A problem that paysites have is intellectual property protection, even with well-secured sites: there are cases of so-called "site rips", i.e. sites are either hacked, or subscribers redistributed content from a member's area without authorization; there have thus been sightings of original content on other TGP sites or entire site rip archives on file sharing networks. Administrators may try to counter these problems by employing copy protection technologies.
Because members must confirm some details of their identity when they make their payment, a touted advantage of paysites is the potential to keep some children and minors from unwanted and/or illegal access. The issue of how to protect these internet audiences from pornographic material deemed harmful to them has been widely debated in the U.S. in light of the 1998 federal COPA act.
Many paysites attract potential paying customers by providing a portion of their content for free via free hosted galleries (FHGs), and use an affiliate program to drive new traffic to those FHGs. The percentage of unique visitors to FHGs who become paying members is known as the conversion ratio.
Source: Paysite Reviews