As “authority to judge was committed” to them, it is to be expected that they will receive cases for judgment. This is what John sees next: “the souls of those who had been beheaded for the testimony of Jesus and for the word of God.” These people have clearly been wronged, a fact that authorities need to state publicly as well as rectify publicly.
4 Then I saw thrones, and seated on them were those to whom the authority to judge was committed. Also I saw the souls of those who had been beheaded for the testimony of Jesus and for the word of God, and those who had not worshiped the beast or its image and had not received its mark on their foreheads or their hands. They came to life and reigned with Christ for a thousand years.