"Koopas Ly" wrote:
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?

Cessnas have the notorious characteristic of not drawing fuel evenly from
both tanks in the Both position. This can result in fuel imbalance. In a
Cessna 172 you might never notice any difference in the flight
characteristics, but in a 182 which has more fuel capacity in each wing
than
the 172 has in total, you can develop an imbalance which becomes fairly
uncomfortable. By selecting one tank or the other, you can burn fuel
(mostly) from the full tank in order to reestablish lateral balance.

I rent the aircraft I fly. Several times the previous renter wrote a squawk
stating: "Fuel gets used only from left/right tank.". Thus far it turned
out that they could not fly coordinated...
Nevertheless, whenever I see this kind of squawk, I select left tank to
taxi, right tank for runup, both tanks for takeoff. After having reached
sufficient altitude, I fly for several minutes first on the left tank, then
on the right. If this works oke, there is nothing amiss with the fuel feed.
Of course, if it turns out there IS a problem, I can select the other tank
(or both) and return with sufficient fuel. Beter to find out early than
discovering later in the flight one of the tanks does not feed at a moment I
do not expect it and possibly at a point which is a bit awkward.