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.
41 Aeronca 65TC Defender
8A7 / Advance NC