Edges are critically important, allowing the eyes to move through the painting as if dancing or diverting the eyes to move in a new direction.

Landscape watercolor sketch

I like to invite viewers to enter into a landscape and move easily from foreground, across the middle ground and all the way back to the sky in the distance. To create this illusion it is helpful to vary the edges as well as connect the values  from distance to foreground.  There must be a way for the eye to travel along light valued areas from close to far as well as traveling along darker valued areas from close to far.  Too often a wall of trees acts like a barrier rather than a screen.  I like to think of the middle ground more as a split rail fence than a stone wall.