There is an essay at usccb.org about why we should read the Old Testament.
Detroit radio priest Fr. John Riccardo points out (to expand one of the points in this USCCB essay) that St. Jerome said “ignorance of the scripture is ignorance of Christ” while he was working on the OT book of Isaiah.
There is an extremely insightful document from the Pontifical Biblical Institute on “the Hebrew Scriptures” here
Cardinal Ratzinger says there that the New Testament doesn’t make sense without the Old Testament.
this document begins to explain why there are OT laws that do not apply to us today.
Marcion was held a heretical view that the Old Testament should be taken out of the Bible along with several books of the New Testament. He didn’t like the “god” of the Old Testament. He said it was a different god than the god of the New Testament. “Reformer” Martin Luther reclassified several books of the OT as not inspired, so his view was heretical, but for two different reasons: 1) Luther only translated into German the books of the Bible from the OT for which he could find Hebrew originals. sadly, he was outsmarted by history, because those Hebrew originals were not discovered until the Dead Sea Scrolls were located. 2) Luther wanted to suppress books of the Bible that were “too Catholic” like Maccabees, which suggested praying for the dead.
One of my personal reasons for reading the OT is, as the late Mother Angelica said, that’s how we find out God’s will for us. Now that I want to quote it, I can’t find the verse that I wanted to use here – We’re not supposed to even mention the name of false gods. Today, I think we know the name of the god of Islam – we should not even say it. (I think that keeps us out of a lot of trouble, too.)