Starks, who underwent ACL surgery in January after suffering an injury during the team's playoff loss against the Denver Broncos, was an unrestricted free agent for the majority of the offseason. Steelers general manager Kevin Colbert never ruled out the possibility of Starks' return, but it clearly hinged on how well his recovery was coming along.
Obviously, the Steelers had to have been in constant communication with Starks and his doctors during the offseason. That partially explains why the team was willing to pick up Mike Adams in the second round of this year's draft—they knew that the odds were good that they wouldn't necessarily have to hand him a starting job right away.
Adams will likely be the Steelers' starting left tackle at some point—and that point could come in his rookie season—but with Starks competing against him, it at least assures the team they have a capable veteran able to play the position if Adams' progress doesn't go as expected.
At the very least, the return of Starks means the Steelers won't have to fall back on the disappointing Jonathan Scott.
Scott started last season at left tackle but was quickly replaced by Starks, whom the Steelers released before the 2011 training camp and then brought back at a lower price. Pittsburgh knew that by having just Adams and Scott at left tackle this year, their offensive line could potentially be in trouble. That's why Starks has been on their mind the whole time.
The Steelers were more than likely waiting for a positive report from Starks' doctors before offering him a new deal. Starks can now pass a physical and should be able to participate in training camp—at least in a limited capacity—as he works to be ready to take the field against the Broncos in Week 1.
Before the Starks signing, the backup plan for the Steelers on offensive line would be to shift right tackle Marcus Gilbert to the left and have Adams play on the right, with Scott and Trai Essex as depth. Now Gilbert can stay on the right, Adams and Starks can battle for the job on the left, Essex can back them all up and Scott can either slip down or right off of the depth chart.
While some teams are heading into camp with under 90 players without any specific plans for those roster spots in mind, the Steelers were at 89 men for a reason. The Starks re-signing was anticipated to come at any point this summer—it was just a matter of the Steelers' being confident in his health.
Starks was the ace up the Steelers' sleeve when they drafted Adams in April. It was why they felt comfortable moving Willie Colon from tackle to guard, and it was why the Steelers' coaching staff could face a stressful situation (a rookie at left tackle) nonchalantly. They always had Starks in mind, and now that he's apparently good to go, the Steelers' offensive line plans have fallen right into place.