What’s the Significance of Self residing in the Equation?

I’m fooling around with metatables and I though to myself, what’s the point of adding Self to Class? As in the example below, you can perfectly run this metamethod, if that’s the correct Terminology, without putting Self into the mix, I’d also like to understand a bit more on Self being an automatically provided argument when writing these functions. Could anyone fill me in?

--// Self applied
local Class = {};

Class.__index = Class

local Self = {};
setmetatable(Self, Class)

function Class.SetString(Self, String)  --// Provided argument?
   Class.String = String
   return Class.String
end

print(Self:SetString("Hello!")) --// -> “Hello!”

--// Self excluded

local Class = {};

Class.__index = Class

function Class.SetString(Self, String)
   Class.String = String
   return Class.String
end

print(Class:SetString("Hello!")) --// -> “Hello!”

Thanks! If there’s anything else interesting that I should know, please provide a link!