Because the lighter the flywheel, the less load you can drop down. Especially on the torque impaired EJ205, that makes a big difference off the line. Also into second since the gears are fairly tall it helps if you can slip the clutch a little. Again harder to do on the lighter flywheel. Flywheels store energy. If you're running around a track at higher RPMs and never really coming to a stop, lighter is helpful since it gets it into revs faster. If you're at a dead stop trying to get into boost, heavier is actually better because you can spin more mass.