Rims need to accurately follow the centre-line of the bike, and it is easily possible to build a wheel accidentally off-centre. The dishing gauge provides the solution. It indicates lateral distance of the hub's lock nut to the rim edge. I usually check the dish (obviously, it must be equal both sides) a couple of times during tensioning. In order to adjust the dish on a wheel that has already been laterally trued, all of the spokes on one side of the wheel are loosened, and the spokes on the opposite side are tightened by the same amount, so as to pull the hub across and centre it under the rim. When the dish, tension, lateral and radial run out are all perfect, the wheel is finished.