I think that Christians often underestimate the value of Mosaic Law. After all we are redeemed from the law (gal 3:13). We are free from its yoke and yet here, the first psalm, the first entry in the books of wisdom talks about the two ways: the wicked and lawless versus the righteous and holy. “Happy are those…[who] delight in the Law of the Lord and on His law they meditate day and night.”
Joshua, the first book in the books of the prophets, has a similar tone right up front. “Only be strong and courageous being very careful to act in accordance with My Law.” (Jos 1:7)
Both of these entries then echo Deuteronomy 6:2-3 which explains the WHY: so that we may be blessed and have long life in the land that we are promised. However Psalm 1 adds a bit to it.
If a righteous person who meditates on the law is like a tree that is blessed by being planted by the water and is blessed to be fruitful, it is because that tree has deep roots that can get to water when it is dry and that can anchor the tree when storms come along. The righteous man is not only blessed by meditating on the law of God, but he is also anchored by it. And even then it is not the law itself that is critical, but the continual revelation of the character of God which the law provides. In much the same way the whole of Scripture reveals our heavenly Father to us, we must dig in and be deeply rooted in that revelation to weather the dry and the storm.