Question

why carbonic acid is added in soft drink

Answer 1

It is added to produce fizz and also has in your stomach

Answer 2

Carbonic acid is added to soft drinks to make it fizzy. When the bottle is opened, the pressure decreases and the carbonic acid changes into carbon dioxide and water making it fizzy, which adds a pleasantly acidic flavor and an interesting mouth-feel if it isn't there then the drink becomes flat and less interesting and less enjoyable (lol)