Comment by nanoxide

It's for streaming on platforms like YouTube or Twitch, usually when streaming games or doing art. Instead of using a webcam to show yourself (mostly in a small overlay in a corner), you show a PNG image of yourself or an avatar. To give these static images for variety, you can change them based on voice input (i.e. as if the PNG was talking), or emotions like laughing. Related: V-Tubers which use 3D models or more intricate methods like motion capturing to animate a virtual version of themselves.