The Shenandoah Valley runs along the Appalachian Mountains from southwest to northeast in Virginia. Besides being a breadbasket of sorts for Virginia and the Confederacy, it was also a perfect invasion and attack route. One Union General called it a "loaded shotgun pointed straight at Washington DC".
For three years that Confederates had time and time again used the Shenandoah to launch cavalry attacks into the North, distracting and diverting Union troops and attention away from the drive towards Richmond, putting the North repeatedly on the defensive and prolonging the war.
To destroy everything in the valley that might provide southern General Jubal Early’s rebel soldiers with sustenance or war materiel was a sound tactical move by General Philip Sheridan (which also proves that he recognized its strategic importance), who wanted to lay such waste to the countryside that “a crow flying over it would have to carry its own provender”.
Sheridan's troops showed no mercy on the land, and it was strikingly effective. After Jubal Early’s final military campaign in 1864, the Shenandoah was permanently closed to the Confederacy and most civilians for that matter.