Why is Satan released in Revelation 20?

Satan is released after the thousand-year reign so he can activate the free will of new generations born to believers. His loosing for a little season also allows him to go out and deceive the nations in the four quarters of the earth, bringing about the end result.

Les explains that Revelation 20 shows Satan bound in the bottomless pit for a thousand years, sealed so he should deceive the nations no more, as he had been doing for six thousand years, until the thousand years are fulfilled. That is the first time Scripture delineates a thousand-year period. After it is fulfilled, he must be loosed a little season. His release is to activate the free will of new generations of children born of believers. Then, as verse 8 says, he goes out to deceive the nations in the four quarters of the earth, and the end result follows.

Revelation 20:7

And when the thousand years are expired, Satan shall be loosed out of his prison,

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What happens when Satan is released at the end of the thousand years?

Satan is released from his abyssal binding and again causes the nations to follow him and reject the sovereignty of the King and of God. This release precedes his final setting away, which cannot occur until the thousand years are finished.