Rest is the most effective and also the most underestimated training strategy.
The training itself is a message that is sent to the body to tell it that a mechanical demand greater than what it normally does is in progress. During training, the body tries to respond to this request. At this moment, there is neither adaptation nor transformation. It is during the rest phase that follows the training that the body transforms and adapts. The training strictly upsets all the physiological parameters of the body and tells it that it must transform itself, which it does during the rest phase.
During rest, the body reconstructs its energy reserves, which allows it to transform its physiology, transform its muscle fibers and transform the way energy is produced inside the cells. Physical improvement reflects this transformation and translates into the fact that for the same intensity of exercise, the amount of energy expended will be less, fatigue will be less and the energy reserves will be emptied more slowly.
If the training periods are too close form each other, the body does not have time to reconstitute its energy reserves or adapt to be more metabolically efficient and improve its physical condition. Such behavior eventually leads to degradation of physical condition and exhaustion.
Rest is essential to improving performance, so rest phases must be a part of your workout to allow the body to adapt and transform itself.