It's weird how effective it is. You take something that's supposed to be innocent, a child's toy, and you subvert that completely. The suspense comes from the violation of that trust, you know? A doll in a rocking chair, staring at a character from across a room, does more to unsettle me than any man with a knife. Maybe because it's passive, it's just there, waiting. The character can try to rationalize it away—'It's just a doll, it's just the wind'—but the audience sees the slight movement, the glint in the eye, and we know better. That gap between the character's denial and our creeping dread is pure suspense fuel.
I read a story once where the doll wasn't even haunted in the classic sense; it was being used as a conduit by a real person, a stalker who would move it around the house to mess with the protagonist. That was somehow worse. The physical object became a symbol of the violation, a tangible marker of an invisible threat. Every time the main character turned her back, she had to wonder if it would be in a different position, watching from a new angle. The suspense built not from jumps, but from that constant, low-grade paranoia that the 'safe' space of home had been poisoned.