We all need help at some stage in life.....
First we have to know why a judgment day is even necessary before we understand what it is.
Why do humans need judging? It goes back to Eden when our first father Adam threw all his children under the bus (metaphorically speaking) by disobeying a direct command from God and becoming sinful and subject to its consequences...i.e. subject to wrong decisions leading to wrongdoing and death. Like an awful genetic defect, we inherited his debt.
God sent a rescue mission in the person of Jesus Christ, but only after allowing the first humans to fulfill the first mandate given to them.....to "fill the earth" with their kind. Each one however was born with the defect of sin, (Romans 5:12) but that in itself was not a reason to lose our everlasting life on account of it. God provided his word and the teachings of his son to guide us to a way that would not lead to everlasting death....the penalty that Adam suffered. His children would be given an opportunity to prove that they could defeat sin by their obedience to God....as Christ had done.
This is what the judgment is for....to see who among mankind can see the value of obedience to God without question, and who believes that it is more beneficial to do your own thing, regardless of what God's says. At this time in history we would clearly see who was would qualify for a place in the Kingdom of God, and who would forfeit their life to selfish ambitions.
If you go back to Eden, obedience is all God ever asked of his earthly children....disobedience took their life away and obedience gets it back, thanks to the sacrifice of Christ who paid the debt that Adam left for his children so that we could become citizens of God's Kingdom on earth.
I believe that doing things God's way rather than our own way is the key to gaining his approval.
The criteria for citizenship is obedience in everything. We can't 'earn' a spot in the kingdom but we can put ourselves in line for it by our conduct. Our entry is up to Jesus who is appointed as judge of all of us. He can read hearts. (Matthew 7:21-23)
I believe that it is necessary because God always finishes what he starts. (Isaiah 55:11) He purposed for this earth to be filled with obedient humans who would represent him here as caretakers of the planet and all the living things upon it....including each other. We have strayed very far from that original prospect, so God set a time limit where he would allow human kind to fill the earth and he would educate them about his ways and then judge them accordingly. But there was a time limit....filling the earth would take time.
Since the Kingdom will "come" ready or not, it is up to us to be found by God obedient to all his commands when it does. We will get little warning about the day and the hour, but the times will alert us to its approach......the world will be in chaos which will get progressively worse. The ruling powers would be pushing against one another.
We have no idea what God is going to do with the rest of the Universe but its vastness suggests that we will not be the only lifeforms in existence....he may well want to fill other planets with living things because he is a Creator who loves to create. I guess we will find out soon enough.
The judgment that Jesus metes out when he manifests as judge will only be for the separation of "the sheep from the goats"...IOW those who are alive at that time will receive one judgment or the other....but there is another judgement day of 1,000 years where the rulership of God's Kingdom will see the resurrection of the dead and their education during that time in preparation for one final test at the end of that period.....only those who pass that final test will inherit what Adam threw away....everlasting life in paradise on earth.
A Grand Millennium Approaching — Watchtower ONLINE LIBRARY
That is how I understand judgment day....