The Sabbath is clearly stated, in Exodus 20 and Deuteronomy 5, to have two major purposes. The Sabbath is to remind us that God is Creator; we look back on Him creating. But it is also designed to show us that the Sabbath is the day that He has given to keep us free; it reminds us that we were once slaves.
Remembering God as Creator is good, but because it happened in the dim past, it does not always help us in our immediate concerns. But every Sabbath we are also reminded that God is our redeeming Liberator, and that we keep the Sabbath because we are free—and because we want to remain free. Those who are redeemed who do not keep the Sabbath do not retain their liberty.
Nations establish memorials for specific reasons. Here in the United States we have a Presidents' Day, Martin Luther King Day, Memorial Day, Independence Day, Labor Day, Thanksgiving Day, Armistice Day, and so on. Why do we have these days? Our nation's leaders want us to be periodically reminded of our heritage. They want us to remember why we have what we have, why we should hold on to these things, and why we should strengthen what we have.
God's Sabbath—His memorial—is so important to His purpose that He has it recur every week! Not once a year, but every week! It is a constant reminder of our spiritual heritage from Him and of our release from sin, and it reorients us in any area in which we may have turned aside.