Whyis aqueous NaHCO3 used for separation of benzoic acid from methyl benzoate?

Benzoic acid is, well, an acid. As such, it'll react with a NaHCO3,which is a base and become benzoate, which is negatively charged. Charged species are soluble in water and other polar solvents, but nonpolar compounds are not. This can be use as a separation

First, add to the mixture NaHCO3. This should convert all of the benzoic acid into benzoate, which will then dissolve readily in the aqueous base solution. Mix it around a bit to ensure that all the benzoic acid is dissolved, then let it settle until the aqueous layer and the organic layer are separated . The top layer is the aqueous layer, because water is less dense than your methyl benzoate. From here, simply remove the aqueous layer with a pipette and be left with a much more pure methyl benzoate solution. You can repeat the washing 2-3 times to be sure, or until you no longer observe an acid-base reaction (bubbling) when you add base.