Fuel tank balance

Typically low wing aircraft "can't" have a "both" setting. Now there
are exceptions (if it has a header tank). So on low wing aircraft
(which HAVE to have fuel pump(s)), you typically have just left and
right. Now I am talking about small GA aircraft, not military or
transport aircraft.

If you have to have a fuel pump, then usually you have two, so you have
a backup if one quits.

As for the mixture, that is very important and often used. Most systems
now have an EGT (exhaust gas temp) guage and you use that temperature
to set the mixture. There is also a procedure for setting the mixture
based on rpm. At altitudes above 3000' (some use 5000), the mixture is
set leaner than full rich on the ground before takeoff at runup.