I once heard a wise man say health = energy (the awesome Tony Robbins). To me this is so true, when I have been at my healthiest I have felt a vitality that was otherwise absent. It gave me more energy to do the things that were most important to me and provided me with more endurance and a greater zest for life.
When you have energy like this you are able to put your all into work and life, making you feel less run down by all that you need to do and more enthused to do your very best. It improves your social life because you are more up for meeting people, the pursuit of your passions is given a boost and generally your life outside of work flourishes. Work wise you feel more motivated and driven to perform well too.