The compensatory picks process isn't designed to create a profit for teams to let their players sign elsewhere in free agency. It can come fairly close, though.
The Steelers got as close as they could to a return on ex-Steelers wide receiver Mike Wallace, gaining the 97th pick in the 2014 NFL Draft, the highest comp pick possible a year after seeing Wallace walk (run?) in free agency.
Only 13 picks and five years separate the 97th pick from where the Steelers selected Wallace, the 84th pick in 2009. Needless to say, lightning can strike twice in that area; it's possible to draft a Wallace again. Theoretically, having the highest pick possible gives the Steelers the best chance possible to select the best player available with that pick.
While technically it's a third round pick, it's more like the start of the fourth round. The Steelers traded up with the Cleveland Browns last year for the 111th pick and took Syracuse safety Shamarko Thomas, giving their original third round pick in the 2014 NFL Draft. They bet on Wallace's departure landing the top comp pick this year, which it did.
The Steelers also added the 173rd overall pick, at the end of the fifth round, for the loss of cornerback Keenan Lewis, and the 215th pick (6th round), presumably for the net loss of Rashard Mendenhall and Ryan Mundy.
Vince Williams, the Steelers' likely starting inside linebacker in 2014, was acquired with a compensatory pick (206th overall). So was David Paulson (240) and Kelvin Beachum (248) in 2012. A team's ability to replace departed starters with new (less expensive) starters through the draft is a key in rebuilding and sustaining any success it might have had. The continued growth of Beachum and Williams, just as two examples, would mean the Steelers get starting level talent for two and three more years, respectively, on low rookie contracts.
Losing a player like Wallace doesn't necessarily make a team better but the salary cap space saved along with the possibility of adding a capable starter in two seasons can be a value-add for any team.