system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do directors start the music or sound effects of the next scene before the video cuts to the next scene?

Is there a storytelling purpose for this? Is there psychology behind it?
gpt
Professional editor here

It's called a J cut (if the audio for the next scene starts before cutting to it) or an L cut (if audio from the first scene carries into the first shot of the second scene.) It resembles the look of the editing timeline with audio running under video. 

It's basically because hard cuts / large visual changes of footage and sound simultaneously are jarring. It's why jump scares are so effective. 

The sound allows our brains to smoothly transition from one location or situation to another because it's a super unnatural thing for our brains to process.

Edit: imagine anytime you entered a new room through a door or got in your car and the ambience of the space changed all at once. The transition or sound of a room or space is natural for us to understand how our state of being and placement is changing.