Why is tartaric acid added into baking soda to get baking powder?

Tartaric acid is added to neutralise the sodium carbonate formed on heating by the decomposition of NaHCO3. If it is not added, the cake would taste bitter due to the presence of sodium carbonate in it. When baking powder is heated or mixed in water, the following reaction takes place,

CO2 produced during the reaction causes cake to rise making them soft and spongy.