It's not always about the victims reactions - this sadistic creature/man from 'The Smiling Man' takes delight and joy in revealing a horrid scene to a naïve girl.
A reaction shot is used to show a characters reaction to something, event, or other character. This is important because it shows the audience how to feel about the event the character is experiencing, or how the character feels and relates to events around them.
In conversations, or when you want to connect two people/objects, you can often bounce the camera between them, this is called a Shot/Reverse Shot. This can be a series of reaction shots of the conversations participants, or reaction shots of people and the person/object/action that is creating that reaction.
Horror films rely on Reaction Shots. Showing the fear on a characters face, builds fear within the audience. This is why the beginning of a horror film needs to connect the audience with the character to build empathy and recognition. Once an audience is connected to a character, the Reaction Shot of fear translates to the audience. Another trick of its use in horror, is when Reaction Shots are held longer than they need to be - this denies the audience information about what is causing the fear, triggering our imagination from a fear point of view.
In 'Stucco', when there's a knock at her door, her reaction by hiding out of sight of the window and slumping to the floor shows the characters fear of the outside world - we can understand that she is experiencing some trauma.
In 'Stucco', when an increasingly paranoid character finds a tooth on the floor, her reaction to check her own mouth shows her doubt and fear that she is losing grip on reality because she can't remember losing a tooth, so why would she think it was hers?
In 'Smiles', our main character has just seen his girlfriends father, who is wearing a mask, and this leaves him very confused. He looks around to check that this is real.
...It turns out the whole family are wearing strange masks. This reaction shot shows his discomfort and awkwardness. There is no explanation for his situation, so his response mirrors, or enhances the audience's curiosity.
In a classic 'bait & switch' in 'The bef', we see the boys fear on his face as the police come knocking - we assume his fear is of the police, only for the audience to soon realise that it is actually the police that are to be afraid.
In 'Banshee' we can see a character lying on the ground. Here, there is an absence of obvious reaction, which is used to create fear, because she is paralysed and unable to move - her reaction is all in her eyes as she sees her dead friend and a creature in the darkness.
In 'Grief', this coversation between a wife (left) and her husband (right) is tense and reveals the fractures they have in their relationship after a tragic loss. The camera bounces backwards and forwards between them showing their reactions to each other - rather than calling this a whole lot of reaction shots, we call this shot/reverse shot - which builds a relationship (a bad one at that) between the two characters.
In 'Kookie', shot/reverse shot happens when the new cookie jar is revealed. This is used to show her reaction (which is shock) but it also builds a relationship between the two - one that is tense, suspicious and obvious in foreshadowing the central conflict. The use of shot/reverse shot works to connect the two, yet highlight their difference.