Unity, VRCSDK, and plugins?

[Monticello]

I have recently gotten into private avatars in VRChat and am having some success and some failures. Whether it be a pink avatar, (I have cubedunityshader but idk what to do with it) or not knowing how to rig certain non-humanoid avatars i.e. Droideka ( 3 feet (star wars)), or Clank ( 3 fingers (Ratchet & Clank)).
But this discussion is about PLUGINS. What is it used for? I look through tutorials and for some reason I've not seen anyone have this folder when they import custom package; this being vrcsdk. When I import, Plugins folder is right next to VRCSDK folder.
Why does this happen? Is it vital to custom avatars? Why does it only show on mine? I'm guessing it's probably just a new version of vrcsdk. What can I possibly do with this folder now that I have it?