John Gill Commentary Deuteronomy 8:1

"All the commandment which I command thee this day shall ye observe to do, that ye may live, and multiply, and go in and possess the land which Jehovah sware unto your fathers." — Deuteronomy 8:1 (ASV)

All the commandments which I command you this day shall you
observe to do
It is repeated over and over again, to impress it on their minds, and to show the importance and necessity of it, how greatly it was expected from them, and how much it was incumbent on them:

that you may live and multiply, and go in and possess the land which the
Lord swore unto your fathers ;
for their temporal life, and the mercies and comforts of it, the multiplication of their offspring, and of their substance, their entrance into the land of Canaan, possession of it, and continuance in it, all depended on their obedience to the commands of God; see (Deuteronomy 19:20) .