This verse shows that the people knew very well what they had to do. They had been carefully instructed by Moses on how they were to serve the Lord. This was true for their time in the wilderness and it continued to be true for their future time in the Promised Land.
We so desperately need to hear this repetition of God's laws—his will for our lives—time and again. We humans are quickly inclined to go our own way and then decide for ourselves what is right in our own eyes. Therefore, it is certainly not superfluous that, for example, the Ten Commandments are regularly recited during a church service on Sundays. It is not without reason that these commandments are discussed in detail in the Heidelberg Catechism (Lord’s Days 34–44). Answer 115 explains the reason for this in detail; in this way, we may more and more become aware of our sinful nature throughout our lives.
And hearing God's will also encourages us to never stop striving and to pray to God for the grace of the Holy Spirit, so that we would be increasingly transformed into the image of God, until after this life we reach the goal of perfection.
17 Mais vous observerez les commandements de l'Eternel, votre Dieu, ses ordonnances et ses lois qu'il vous a prescrites.