Why is local used in front of functions in some tutorials but not others?

So I have reading and watching tutorials and I have noticed something odd. Some of them use local in front of function like so:

local function helloWriter()
	print("Hello")
end

while others just do this:

function helloWriter()
	print("Hello")
end

Could someone explain what is going on here? I know what local is for when using variables however I am confused by using it in front of functions. Thank you.

If i'm not mistaken, it's the same difference as with variables.

Probably helps with loops or something.