In this super useful video by the good folks over at Phlearn, we delve into what separates 'Curves' and 'Levels' adjustments in Photoshop and when to use one over the other. These reasons 'when to' and 'when not to' use aren't the end-all-be-all, but you will definitely walk away with a better grasp on these two types of adjustments after watching.
To recap the video....
The best time to use Curves is when you are trying to make more aggressive changes to the image, adjusting the colors of shadows, midtones or highlights and adjusting very specific values. This is my personal goto for most of my color and tone correcting when I need a more specific change.
The best time to use Levels is when you need to make subtle changes in tone or color or easily adjust the absolute white or black values.