Undervolting Purposes

So, quite often in r/AMDHelp one of the first major suggestions to people experiencing issues with their cards is asking if they've undervolted their GPU. I'm fairly inexperienced when it comes to proper or in depth tuning of computer components and I never really understood why this is such a common suggestion.

Basically I'm asking why undervolt? Why is it such a common suggestion and what exactly are the benefits of messing with voltage?

I've gone through 4 different AMD GPU's (RX 480, 5700xt, 6700xt, now a 7800xt) and I've never felt the need to do anything of this sort, but I'm trying to expand my knowledge in this department and experiment with my hardware and see if I can weasel out some extra performance. So please share your knowledge, all holy internet wizards.