Notwithstanding the fact that no one here seems to have seen this on a C152, I'd venture that it can be useful on a C172, which does have a L and R option as well as a Both option. For example, a rough running engine might be cured by selecting L or R if the cause of the rough running is fuel contamination one tank only. Also, the POH recommends setting it to either L or R when parked - leaving it on Both will cause some cross-feeding due to gravity, and the tanks will not be balanced when you return to the aircraft (I don't know how, or if, this is avoided on a C152 which only has Both and Off positions?)
FFF
--------------