Audio Interfaces are, in most people's case, a high quality alternative to recording directly into the microphone line of their computer
-One advantage is that you (almost always) have an XLR input, meaning that you can easily use a high quality condenser or dynamic microphone in recording.
-Another is the 1/4 inch inputs, which allow for easily recording the outputs from stringed instruments such as guitars.
Aside from those you will get gain controls, sometimes an EQ, phantom power, and various other things such as insert channels or monitoring outputs. The main point of getting an a USB input into your computer is so you can avoid the noise from your computer's soundcard and ADCs / DACs (sorry I forgot lmao), but also to have access to (hopefully) higher sample / bit depths from recording. The main reason that people usually get USB interfaces as opposed to USB mixers, is that mixers will generally always give you a stereo input while an interface is multi-in, meaning that you can do multitrack recording in your DAW.
