The third function of the commandments is to be a guide along the path of life, to show us how to live. So God blessed the seventh day and hallowed it, because on it God rested from all the work that he had done in creation. Later on in Exodus chapter 31 we read another account of the giving of the Sabbath Law. God told Moses to tell the people of Israel to rest and not to do any work on the Sabbath day, because the Sabbath was a sign of the covenant between God and his people throughout the generations, and a reminder that it is God who sanctifies, and makes them holy. God wasn't tired, but he wanted to model for us a rhythm of rest and work, which he knew we would need, in order to be creative people, made in God's image.
Rest is essential to the maintenance of any kind of balanced life.
Observing the Sabbath was a very serious matter for the Israelites. The punishment for profaning the Sabbath was death. It occurred to me that this is one way we are counter cultural, when we honor our relationship with God by coming to worship on a Sunday. There's no rush hour to avoid on a Sunday morning as we drive unimpeded to church, with so few cars on the road. In Judaism observing the Sabbath and circumcision, became the two essential practices of their religion, which distinguished them from Gentiles.