There is a balance when training between being rested and therefore being able to work harder in a given workout and accumulating enough cumulative training stress to become fit. Most of the time you train with a certain amount of fatigue present because that is the best way to reach your big picture goal- say an A race or an endurance event or whatever.
Prior to your goal race/event, you rest to remove the fatigue and then you see the gains. It's impractical to be fully rested for every single workout, the art is being rested enough to get the work done.