With regards to fuel tanks in a C172, why does the takeoff and landing
checklists both call for the fuel selector handle to be in the "both
tanks" position?

The only time that fuel is set to one particular tank is on the
ground. I've never used fuel from only one tank in flight. Why would
someone do that?

Not really related to 172's, but our RV-6 has no "both" position... either
left, right, or off. You just have to remember to switch them occasionally.

Also, IIRC the 150 has just "on" and "off".