It's different in "Stranger Things." Normally a flashback is used for pure shock or as a back-story reveal to inform the audience of something. The flashbacks in "Stranger Things" howeverare used to put you inside the character's head at a precisely "triggered" moment.
Their function is to increase the stakes of the drama.
Interestingly, the flashback is catalyzed by something in the present moment which "triggers" the character into diving into their memory.
It's not a simple drifting away into one's mind, or simply into the past, as is usual in a standard flashback scene.
In "Stranger Things" something happens in the present which mirrors an event that happened to the character in the past. The flashback is linked to the present via this mirroring effect.
Time stops in the present moment as the character is remembering the event via flashback. We experience a pivotal backstory moment of the character that significantly raises the stakes of the present-frozen-moment. After the punch of this effect, we're brought WHAM back into the present moment. Time continues.