The San Francisco Giants were reluctant to play Jackson or Gearrin in any kind of meaningful role. As a result, they held on to these players on the active roster. It created a roster crunch. Trading away Jackson and Gearrin creates some roster flexibility.
Jackson came into this season as the starting center fielder. However, after demonstrating limited range, he quickly found himself on the bench. To make matters worse, Jackson struggled to make contact. He was striking out 35 percent of the time. That is kind of a lot.
With his struggling defense and poor hitting, Bruce Bochy had difficulty in finding a role for him.