Why will there be a judgement day?
Jesus has offered us a way out of sin, so that it doesn't last for eternity. There are people who have rejected his offer.
There will be a time when those who refuse to part ways with their sin will be judged and their time will end because it's the only way to get rid of sin.
If God let's people keep their sin, it will continue to infect everyone. Much like a someone with a virus around healthy people.
Eternal life belongs to those who are willing to let go of the authority sin and choose Jesus instead.
Matthew 25:31-34 " “When the Son of Man comes in his glory, and all the angels with him, he will sit on his glorious throne. 32 All the nations will be gathered before him, and he will separate the people one from another as a shepherd separates the sheep from the goats. 33 He will put the sheep on his right and the goats on his left.