It was actually written for a variety of reasons. First, to tell of the Jesus' life and death, and why they are important (The Synoptic Gospels), to detail the acts of the apostles of the early church (Acts), to instruct people in early churches (I and II Corinthians, Galatians, Ephesians, etc., written by Paul to the various churches he established to direct them), and to give the believer a glimpse of the rewards to come (Revelation, written by John of Patmos).
One of the main reasons of the New Testament, though, is to complement the Old. Though we may read the Old Testament and finish it with a sense of satisfaction, we shouldn't, in a sense; the story is not over.
Adam's fall introduced sin into the world, and committing a sin always carries a consequence that counterbalances the sin with justice. Because of the fall of Adam, humanity is still tainted with sin, and because sin carries a consequence, humans must always pay an immediate price for committing a sin to balance the scales of justice. In other words, once someone committed a grave sin that took more than a "I'm sorry" to atone for, God had to punish them swiftly. That way, the price of sin could be paid quickly. Another way to pay for sin was through the sacrifice of an animal without blemish (very important that the animal is flawless; in the Old Testament, this represented something without "sin."). The above reasons are why, in the Old Testament, we have many sacrificial laws (for example, a person who stole has to pay it back with 20% interest and then pay for a ram to sacrifice to atone) and many steep punishments (such as stoning for witchcraft).
There are laws that call for death for certain sins or an immediate monetary payment for others. Because there was no way for God to pardon a person for sin without punishment, sacrifices and strict laws had to be enforced so that justice could be served and sin could always be paid for.
So, how could God give humanity another chance without their having to pay for the sin immediately? The only way was for a man to do what Adam did wrong: have a man who, unlike Adam, lived a life completely without sin (not even petty sins, such as lying, etc.). So, God was working on it.
The Old Testament was never over because prophecies foretold the coming of such a man: a man who would be named Wonderful, Counsellor, the Mighty God, etc. This man would live a sinless life, and, much like a sacrificial offering of old, sacrifice himself to atone not just for a temporary sin, but for the sin of all of humanity permanently.
Since the Old Testament did not record the arrival of such a man, the story was not complete. So, Jesus came. As Son of God, yet, Son of man, he was a human being, but a flawless one, who was not sinful and never committed a sin. He was perfect.
Therefore, when he died on the cross, he sacrificed himself. Since he was without sin, only he (like the animals without blemish) could be sacrificed for sin. And so he was. Because of this, he could bear sins, the sins of the entire world, of all humanity who ever lived, taking them away from humanity permanently.
Which leads us to: what about the sacrifices and rituals? Gone. The sacrifices were used to atone for sins temporarily. Now that Jesus did them permanently, all the sacrifices of the Old Testament were unnecessary. Jesus "fulfilled" these laws, meaning that his permanent sacrifice permanently fulfilled the requirement of atonement in the Old Testament for every sin, forever. Now, to be cleansed of sin, one only needs to ask Jesus into his or her life and ask for forgiveness, and his sin would be atoned for.
That is why all sacrifices and rituals are not necessary. That's why some parts are now not obeyed.
As for who wrote the Old Testament, a lot of people did. Moses wrote the first five books of the Bible (Genesis to Deuteronomy), Joshua wrote the Book of Joshua. There are many other authors, such as the prophets Isaiah and Jeremiah, Nehemiah, who rebuilt the wall of Jerusalem, David, who wrote the Psalms, Solomon, who wrote the Book of Proverbs, and Micah. Authorship is disputed sometimes, but all Christians believe that these otherwise unrelated authors were inspired by Jehovah.
Blessings.