It's a bit of both. Closed species are a great way for certain artists to market their niche species/concepts to a group of dedicated fans who theoretically share similar aesthetic or worldbuilding tastes in an insular, cohesive narrative; or, at least, that's the idea. A lot of the people buying these species are hoping to befriend fellow artists (specifically those they admire) in the hopes of interaction, whether through roleplay or storyworld discussion or what have you. Throwing money in to buy a specific design (or multiple designs) is, to some people, a much more plausible way to catch the attention of someone you admire as opposed to just throwing money at them on Patreon in hopes of getting their name listed under Thank You credits.
The messy part is where money gets involved and the bigger/more popular a group gets the more hands-off an artist is going to become, instead relying on staff to run things and essentially becoming a mini-business. This is the point where closed species fans tend to get toxic; they form their own friendships and that's fine, but they'll also start seeing anyone who isn't participating or seeming to 'rip off" their species with an "us vs. them" attitude, especially if they've sunk a lot of money into these groups for the prestige of having the artist actually remember who you are. At this point even if the original artist's intent is still good, they need to step in at some point to tell their fans to calm down otherwise things will escalate beyond their control; they might not be the only person running the group, sure, but the responsibility for group rules still lies on the leadership in my opinion.
For people who created their closed species specifically for the prestige rather than out of genuine enthusiasm for their story concept or to have something small to share with a handful of close friends, this can breed a kind of narcissism in which the creators of these closed species groups will actively use their fanbase to harass other people. As mentioned above, the responsibility for the group's behavior lies largely on the creator(s); if individual or a group of members are behaving badly then by all means kick them out, but it needs to be done on an ethical basis rather than a personal one.