God gave his people the Sabbath for their good. It was a day on which they could rest their bodies from physical labor and refresh their hearts and minds by remembering his works of creation and redemption (Ex. 20:8-13, Deut. 5:12-15). J. G. Vos asks a good question – and answers it well in his commentary on the 4th commandment:
Q: Why does Satan, with his servants, try so hard to break down and destroy the Sabbath day?
A: Satan, with his agents and citizens of his kingdom, is engaged in an age-long warfare against God and God’s kingdom. God’s kingdom is a spiritual kingdom, and it is defended and propagated by spiritual weapons and methods. The real extension of God’s kingdom depends on people’s being converted to Christ, repenting of their sins, and loving and serving God sincerely and loyally. These things depend chiefly upon the preaching of the gospel and the public and private exercises of God’s worship, such as Bible study, the sacraments, and prayer. These divine ordinances can find but little time on weekdays; they are largely dependent on the Sabbath day for an adequate amount of time and attention.
Satan of course understands this, and he realizes that if he can break down the Sabbath, then the preaching of the gospel and the ordinances of divine worship will be neglected – if the preaching of the gospel and the ordinances of worship are neglected, then God’s kingdom cannot prosper – if God’s kingdom cannot prosper, then Satan’s kingdom will not be interfered with – and if Satan’s kingdom is not interfered with, then Satan will have a clear track to accomplish his wicked purposes in the world. So we see that the Sabbath, far from being an arbitrary or unreasonable command of God, is calculated to accomplish a great purpose and to form a real bulwark against Satan’s kingdom and the floods of iniquity.”