A "both" position doesnt work on a pressure feed system, as starvation in one tank would cause air to be drawn into the system. Its OK in a gravity system, where there is no need to pump from the tank. The checklist you have is simply ones perception of checking fuel feed from both tanks...its not a nessesary function. I start,taxi and take-off on the same tank (usually the last one a flew on),and seldom ever select the "off" position. Just my preference, as it is the least amount of change from the prior flight where all was working fine.
8A7 / Advance NC