Jameer Nelson's there just to match salary. His main value to the Celtics is probably to show what Stevens' system looks like with a PG who can hit threes, something Smart and Pressey have not (and may never) do. He's making a decent 36.9% of them right now, close to his career average, while 63% of his shots are threes, well above his career average.
He's going to be a bit difficult to move because he has a player option for next season, unless he looks like he will get more than the minimum if he declines his option.
Wright is atrocious at post offense and not so great at post defense, while being a poor rebounder. He's good at other stuff so that he looks great according to advanced metrics. You probably need to make heavy use of smallball lineups to get maximum effectiveness out of him.