Sefer HaChinuch ספר החינוך

That the king not amass much silver and gold for himself: That the king not amass a plenitude of money — meaning to say that he only have under his hand the amount of what he needs for his chariot and his designated servants. And about this is it stated (Deuteronomy 17:17), “And much silver and gold, shall he not amass for himself.”

From the roots of the commandment is that which is elucidated in Scripture, “That his heart not get haughty, etc.” (Deuteronomy 17:20).

From the laws of the commandments — that which they, may their memory be blessed, said (Sanhedrin 21a) that it is for himself that he cannot amass much, but he can amass much for the sake of Israel and for their benefit, meaning to say to protect them and their cities and their places from the enemies; and the rest of its details — are in Sanhedrin (Chapter 2). And a king that transgresses this and sets up his plan to fill his storehouses with money only to fulfill his desire — without his intention being to benefit the people and their defense — has transgressed this negative commandment. And his punishment is very great, as the entire people depends upon the king. Hence he needs to place all of his attention to the good of his people, and not to his own glory and the fulfillment of his desires.