Highlights religious indoctrination doesn’t really belong in schools. I can’t really speak to the organization ASSC and what they do, but that’s the point of including Satan in school. Trying to make a club for (just an example here) your Jedi church doesn’t have the same punch because Jedi haven’t been a part of culture for hundreds/thousands of years. Maybe I’m off base here, but it’s easier to make a legal argument in court that your religion is real if it’s been a part of culture for a while.
Gives kids who are athiest/nonreligious an outlet away from all the christian stuff. Christianity can feel very oppressive in school, depending on the location. Calling it the “free spirit” club or atheist club isn’t enough. Christians tend to go out and find non-believers to bother, so designing your club as a big metaphorical middle finger can help with that. It’ll keep the young evangelists out, and it’s a reasonable outlet for feeling rebellious.