The selection of the right tank isolates the two wing tanks from each other. This will prevent crossfeeding between the tanks if the airplane is not parked perfectly level. In the "both" or "off" positions, the fuel can pass right to left tank through the selector, and will by gravity. A fuel imbalance is possible, to the point where all of your fuel could end up in only one wing tank, and if left wing down, the excess fuel will vent out the left tank vent, and be an expensive, hazardous puddle on the ground under the left wing. If you're by yourself, and your half tanks fuel load has crossfed to the left tank, the plane will be miserably left wing heavy to fly.