system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] Why are planes' engines slightly tilted down?
gpt
They are tilted down and also left or right.

The reason is that: 

engines do have a torque and the torque will try turn the plane left or right. So you point the engine in a way to cancel a bit of that.

Engines will pitch the nose up the more you ask power from them, they are tilted in a way to counter the nose up they give. 

These angles have the purpose to cancel the effect of engine thrust, this makes the plane deviate less when the pilot changes the throttle, and makes the plane save fuel because you need to use less flight control surfaces to keep it straight.