Why "observe" Shabbat?
To remember we were slaves
And G-d brought us out
Observe the Sabbath to keep it holy, as Adonai your G-d has command ... You must remember that you were slaves in Egypt, and Adonai your G-d freed you from there with a mighty hand and an outstretched arm. It is for this reason that Adonai you G-d has commanded you to keep the Sabbath.
But in Exodus
We're told: "remember" Shabbat
Because G-d rested
Remember the Sabbath to keep it holy ... For in six days G-d made the heaven, the earth, the sea, and all that is in them, but G-d rested on the seventh day. G-d therefore blessed the Sabbath day and made it holy.
Much has been written about the two different versions of the Fourth Commandment. But I think there's meaning in which command (observe or remember) and reason (redemption or creation) are used together.