So why is the old testament even in the bible if it's apparently so damn irrelevant?

okay since you don't know how Christianity works or the bible the old Testament is show us the past the history of the world but also to show us how the sacrifice of Jesus changed the way Christianity was done i.e no more animal ritual sacrifices stoning people for adultery ect. infact any law in the bible which commands another person to kill someone is now outdated since Jesus said not to judge and judgment would be held for god alone when a person dies. so summery old testament is a book of history full of old jewish laws and the beliefs of Judaism which became irrelevant when jesus said judge not least ye be judged

it provides background. The OT has a lot of history of God and the Jews. It also has genesis