Samuel E. Waldron, A Modern Exposition of the 1689 Baptist Confession of Faith, pp. 272-276:
III. Its Appointed Day
The Confession first mentions the natural necessity of an appointed day of worship. The Confession teaches that the law of nature requires an appointed day for worship. First, God must be worshipped publicly and corporately by men. Second, such public and corporate worship requires a publicly and corporately agreed upon proportion of time. Such a proportion of time must be appointed by God, because the only alternative (that men should appoint it) would violate the prerogatives of God in his worship.
Waldron hits the nail on the head! Do you love to worship God corporately with your brothers and sisters in Christ? Then you need a day wherein you can do that. That is why God gives us a Sabbath! The Sabbath was made for man, not man for the Sabbath. It is a gift to all of those who desire to worship Him corporately. It is a natural necessity because without it, there will be no regular corporate worship.
BTW, Andrew, where did you obtain an electronic version of Waldron's book?