When we sleep another of the benefits of sleep is that our body goes into repair mode. It focuses on repairing and building bone, muscle and other tissue as needed by our body. The body is set up to go into a sort of maintenance mode while we sleep that allows it to concentrate on fixing things. Not that our body doesn't do some repair while we're awake, but sleep allows it to concentrate without dividing energy between repair and all the tasks we perform when we're awake.
Mental well being is also one of the sleep health benefits we get from rest. Getting proper, regular sleep helps the mind to unwind, regroup and cope with stress and other issues. When we don't get our rest both body and mind are less able to cope with and adjust to stressors. We are also more prone to developing depression, anxiety and other illnesses. Sleep helps to keep the chemicals in our brain balanced and allows our mind to process out stressful or painful things from our day. We miss out on these sleep health benefits when we don't get our rest.
It is hard to recoup lost rest and get back the benefits of sleep. If we miss a couple hours a night during the work week we can't necessarily just sleep longer on Saturday and expect that we've balanced out our lack of rest from the week. Most likely you will need a little extra sleep for a few nights in a row for your body to feel back on track.