I see the law as being God's technical document.
And when Jesus, who was full of grace and truth, says something (in the gospels),
and even when Paul says something,
and I want to understand their saying, I go to the law to see what they are talking about.
The technical document helps me to know what Jesus and Paul were speaking,
and in turn, Jesus' and Paul's sayings help me to understand the technical document, which came by Moses.
Of course, the words of the OT prophets are extremely useful (probably essential) to understanding the technical document also.