Let's just understand this way. If you look at any sound that's generated through a computer is in the form of a digital signal. That signal is later converted into an analog signal by the inbuilt sound card. The process is very quick and requires a lot of power to do so. Using any computer's sound card will reduce the quality of the vocals or instruments that are recorded. Here comes the audio interface. The audio interface reproduces sound signals that are much more accurate and the reduction in the quality also gets decreased.