John Gill Bible Commentary

For it is not a vain thing for you,.... That is, the law is not, and the observance of it in them and their children; it was no light and trifling matter, but of great importance and consequence, obedience to it being attended with rewards, and disobedience with punishment: because it is your life: if obeyed, the means of a comfortable and happy life, in the enjoyment of all good things, of the preservation and continuance of it to a length of time; and long life was always reckoned a great temporal mercy: and through this thing ye shall prolong your days in the land whither you go over Jordan to possess it; that is, through their obedience to the law they should live long in the land of Canaan, which they were just going to possess; which explains what is meant by its being their life; their comfortable living and continuance in the land depended on their obedience to the law; see Isa 1:19.