Fandoms are good because they create a sense of community
Fandoms allow people who share similar passions to bond with one another, ultimately providing them a place to thrive and build social connections. They can also create a space for people to freely express their ideas, granting them the potential to affect change.
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Members of fandoms support one another by creating a space of inclusiveness. Where a person might not have felt like they fit in before, becoming part of a fandom would make said person feel like they were part of a community. Fandom culture is participatory, this aspect of the community makes it more than just having shared interest. The participants' shared interest brings them together, but their relationship is solidified and continued through contact over the internet and other mediums.  Furthermore, a byproduct of participating in fandom culture is that it can help a person develop and/or further build upon their own identity. Since individuals are circulating the same ideas and passions, these ideas become reinforced, also serving to tighten the bond between fandom participants. That bond would translate into self-sustaining connections over mediums of communication, like social media, where participants then experience a sense of security akin to being part of a family.  Having that sense of security in one another then becomes the foundations for that fandom culture's community, where like-minded people may join and become a part of the community, and somewhere along the line feel the inclusion they sought.
Despite the inclusiveness fandoms appear to champion, there exists a catch twenty-two. Due to the way fandoms are centered around a particular figure or franchise, this can lead to what's called a Geek Hierarchy.  These occur as a result of fans having varying levels of investment in a fandom or the ways in which a participant expresses how deeply immersed they are in the fandom. Where a fan falls on this spectrum of immersion can then lead to fans casting judgment upon one another, creating an air of exclusivity. The ability for fans to judge each other this way attests to existing stratification within the community, which can serve to further alienate people within the fandom as well as people coming into the fandom.