The idea of keeping challenges balanced is avoiding a task from becoming boring and/or stressful. We need balance in the workplace to ensure that our own skillsets are being met as well as feeling stimulated.
People, in general, want and need to grow and expand their own range of abilities. If they do not have the opportunity to do so, they will lose interest in their work and may look for challenges elsewhere.
Balanced challenges allow employees to get their work done, provide a better performance, enhance critical thinking, and innovative problem solving.
This level of balanced challenges helps employees enter the flow while working and growing their capabilities. New challenges will push their boundaries to the next level and intrinsically motive them.