Every good forum has some threads like the Groan thread. People know they are for entertainment and if they want to read them they can. It's like recess for kids. Personally I checked it regularly because there were lots of funny memes etc. All forums need traffic and there isn't a great reason not to allow a thread or two that are off topic. The best forums allow people to express themselves outside of the "serious, on-topic" threads. Alternatives are people putting memes in regular threads or more likely avoiding the forum altogether.