This period allows the plant to rest and regrow leaf area. The rest period is important. If plants are not allowed a rest period cattle will continue to regraze the plants that are still in the vegetative stage. These plants may not get an adequate rest period to build up nutrients that can supply the roots. The root mass is dynamic, constantly changing.
Roots grow, die off, and regrow again.
If the plant does not have adequate leaf area to replenish root growth the plant will have a smaller root mass. This is why it is important to allow an adequate rest period during a drought. It is better to over graze just a small portion than to over graze the entire pasture. Managed grazing systems attempt to accomplish some of these things. The level of intensity depends upon the producer's goals.