Purpose of gain knob above fader ?

This is one of the most important knobs, you're ever going to play with lol true story. It's as Sharke suggests, it's used for fixing your gain. But you can use it more broadly, however, for example before you begin a mix you could make sure every track is sitting at roughly -18db. This is a good way to learn mixing if you're a newb. Another use, altgough I use Wave's gain knob 😎 in their NLS plug, but I also use the gain knob 🤣 to finalize the level for the master buss. It's almost impossible to not go over the 0db mark otherwise. Mixing, and creating headroom is also key to getting bigger mixes too. Ignore this advice if you mix and master and send signals outside the box when doing so. You can also use the gain knob 😎 to set levels to Aux and Buss tracks.