Obviously, this particular trade is good for Chicago because it clears the way for Williams (or whichever quarterback they draft) to start immediately (if that is the direction they wish to take). Barring any other trades involving the Bears in the first round, they also retain the ninth overall pick which they can use on a wide receiver.
The additional second round pick they receive from the Steelers would allow them to address other positions of need early in the NFL Draft. PFF projects that they would use that pick to draft Penn State edge rusher Adisa Isaac.
Most importantly, the trade would mean the beginning of another new era in Chicago with a quarterback who could easily be the best they have ever had.
(Not that the bar is set that high)