RPG forums represent a disproportionate population of gamers. If forum gamers are a vocal minority, they can also be an influential one that can impact
how the game is played even amongst casual gamers. How? Forum readers. We live in the era of the video game guide. Have a problem of how to play? Look it up online. Read it in a class guide. Or watch the solution or some tips and tricks on YouTube. Options you want not available? Look for some online (DMsGuild, homebrew tumblr, etc.).
I have encountered players and GMs who I know do not post on forums discuss with me things they have read online regarding D&D and other tabletop RPGs. Here's a homebrew they found. Here's some advice they read. Here's some options they are not permitting because they read how it can break the game. (I've also had the surreal experience of some of these people, who don't know I post here, talk to me about threads in which I have participated.

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It just takes one person who reads the gaming discussions online to influence the table, whether they are GM or player. This impacts play experience.