Play for the story and the creepy atmosphere. No horror game excels Amnesia: A Machine for Pigs in those regards.
Note to anyone who has played Amnesia: The Dark Descent:
Please do not take this game as a sequel to Amnesia: The Dark Descent. What many people do wrong with this game is that they go with the mindset of the previous Amnesia - going for the horror.
Now, I'm not saying it's wrong to play horror games for the horror in them, and it definitely is not a surprise why people think of this as a sequel, what with the blaring "Amnesia" in the name. However, while the game does make an occasional reference to the original, it's significantly better to think of this as a stand-alone game.
Amnesia: The Dark Descent was made by Frictional Games. They have made a name for themselves by bringing back the horror in the horror genre. Their games have that creeping atmosphere that you can never be rid of. From Penumbra: Black Plague onwards, they took a theme of defenselessness, which definitely has been a big asset.
Amnesia: A Machine for Pigs was made by The Chinese Room. When Frictional Games thought about making a sequel to the Amnesia series, they decided to have an other company make it instead. Being impressed by Dear Esther, they approached The Chinese Room for sharing similar mindsets in their game development. Frictional Games did not want The Chinese Room to make a game that is similar to the original, they wanted them to make a game in their own way.
Two different companies make a game of the same world, with the same theme. It's obvious there'll be differences.
From Amnesia: A Machine for Pigs, you'll get to experience a creepy, gross, horrifying story of a mad man trying to redeem himself.
From Amnesia: The Dark Descent, you'll get a full-blown horror blast in your face. Scrreeeaaaam.