The reason we must obey the words of God without adding to or subtracting from them is that there is the will of God for our salvation in every word of God.

The Sabbath is the commemoration day of the Creator who rested after creating the heavens and the earth for six days.
The Bible testifies that God who created everything in the world is Elohim, that is, God the Father and God the Mother. To let us know this fact, God commanded us to remember the Sabbath by keeping it holy.

Keeping the Sabbath holy is the sign of God's people; and those who desecrate the Sabbath will face the punishment of God's wrath.