Supernatural elements in the wrong place of the novel

I like to read adventure stories. recently, i read King Solomon's mine. the story is exciting. but why did the writer add some mysterious and even supernatural elements into the story. i mean Gegel, the witch. the writer hints  that she was 300 hudred years old.Her apperance was very"mysterious" too. This create some terrible astomphere to the story. This made an adventure story not so reality.It is trouble when i introduce the story to some children since Gegel may frighten them.

Many traditional chinese novel had this problem.supernatural element always appear in detective and even historical novels.Spirit of the dead general would rise up to resuce his nation. The spirit would also tell the detective who had murdered him or her.This was crazy.Unless we expect this was supernatural detective(such as the tv drama of Medium ,Tru calling.)., or supernatural historical novel(such as the story of the Trojan war,King Arthur and the sword).

what is your view?