More examples: #306876 | Isekai Kita no de Mahou o Sukebe na Kot… | 53,054 page 9 #390349 | Keppeki Fuukiiin Succubus no Internship… | 16,144 page 19 #636055 | ゾンビっ娘の救済は中出しSEXで 1~10 | 871 page 169

More examples: #635534 | Succubus ni Chinpo Dake Tensou Mahou de… | 7,466 page 2 #612943 | Ishuzoku Lesbians - Interspecies Lesbia… | 1,710 page 19 #335958 | Louise ga Shoukan sareru Hanashi | The… | 6,050 page 4

It's a niche. I agree it's mainly a feature of magic/fantasy, but not necessary. For example ##522655 | Jikan Sousa Nouryoku o Motsu Taimashi-c… | 6,532 | Jikan Sousa Nouryoku o Motsu Taimashi-c… | 6,339 doesn't go well with fantasy tag. I personally searched for magic circle in multiple sites and never once search for "drill hair" but perhaps there are more people into magic circle theme then we thought?

but then it's just a magic/fantasy setting, why be so specific?

It's a setting, like crown or tiara. Keyword itself doesn't necessary have to be a fetish, but it can help if you are in a mood for a particular setting.

my real question is, why should we need it? It's basically just part of "fantasy" settings which has already been proposed as a tag. In your examples there's nothing worth of note. "tentacles" is already a tag, "wormhole" too. Do people get off to magic circles or something?

More: ##647355 | 2D Comic Magazine Akuochi Shoujo ni Ako… | 568 page 68: used during spell casting ##351911 | Ana to Muttsuri Dosukebe Daitoshokan 3 | 3,804 page 12: used as portal/wormhole

Examples: ##522655 | Jikan Sousa Nouryoku o Motsu Taimashi-c… | 6,532 : whole chapter involves using magic circle as defense mechanism against tentacles ##653441 | Saimin Mahou de Sokuochi❤ Namaiki Majo… | 6,179 page 18: background but relate to the context I'll add them to the list as they show up