These interfaces vary a lot in size, price and function and the right USB audio interface will vary based on what you need, how many inputs and outputs you are looking for and what mics you are using.
While many computers do have good internal built-in sound cards, these external devices are specially built to capture robust audio and have mic-preamps built into the XLR cable inputs that will get you better sounding audio than an XLR mic typically does. If you are deciding between purchasing a mixer and a USB audio interface, remember that a mixer does not actually record your audio and would still need to be connected to a laptop or other recording device, so in some cases an audio interface can be advantageous. USB audio interfaces vary in quality, features and input options but will almost always give you a better sounding audio file than recording with a microphone that connects directly to your computer with a USB. Fulfilling a similar function as an audio mixer in some ways, a USB audio interface can allow you to connect any microphone to a computer or iPad to record audio files and handle multiple tracks at once.
