Pinterest and some other communities has a feature where visitors can read for x minutes/seconds before there is a popup that require registration to continue reading.
The goal of the forum is to get people to communicate. Kind of like forcing people standing outside a networking event to step into the venue. Some people will of course get upset and refuse to come in and open their mouth. Im guessing some people will even find it rude that they are forced to contribute to the event.
Other communities might have other goals. Each to their own.