One epic use for a forcefield I once managed was in a 2v2 game with my ally being a terran, we thought we got the upper hand against a pair of zergies, but one of them had a huge force of zeglings hidden to the side of the map while me and my ally were busy assaulting with mostly siege tanks and immortals (other stuff was inbetween but they were just support), I had a squad of sentries inbetween my forces, so I saw those zergling and the terran paniced and undeployed the tanks, but I quickly planted the forcefields around his tanks, blocking them in, he redeployed them and he continued to blast at the base while I took care of the zerglings with a mothership.