Fuel selector question

Sorry if this is dumb (I am only at 8.3 hours toward my PPL)...

In the email I got from AOPA ePilot training tips today there is a
discussion of fuel starvation that makes regular mention of the fuel
selector switch. All my flying is done in a CE172 and the first and
last time I worry about the switch is during pre-flight when I confirm
that it is on both. I fly on both and never fly any other way and to
be honest, outside of gravity flow issues when parked on an incline I
cannot think of why one would want to select one tank at a time vs
both.