What is returning useful for?

Basically the return command will take you out of the function immediately.

function eatCheese()
    if 2+2=4 then
        print("woohoo")
    else
        print("I hate life")
        return --ends the function
    end
print("function is also done") -- could also end here if 2+2=4 is true
end

If you specify an object, such as a boolean with return, it will return that boolean. So basically, it will also set the function equal to what you returned.

function eatPizza()
    return true  --returns the value true
end

if eatPizza() == true then --eatPizza() returns true, so it would work here
    --content
end

returning in a function allows you to use the function like a value. Examples:

function mathstuff(x)
    return 2*x + 5
end

print(mathstuff(3)) --Using the function as a value.

function equal(a, b)
    return a == b
end

if equal(4, 2 + 2) then
    print("2 + 2 = 4")
end

function a(text) --Declare a
    return "MWA"..text --Add "MWA" to the beginning of "HAHAHA".
end

function b(text) --Declare b
    return a(text:rep(3)) --Repeat "HA" 3 times (or twice, technically) and send to a
end

function c(text) --Declare c
    return b(text:upper()) --Make "ha" uppercase, send it to b
end

print(c("ha")) --Call c, "text" in c is "ha"

--Returning nil, or just returning with nothing there,
--basically means you're cancelling out of the function.
--This lets us continue on in the script without errors.

function square(argument)
    if type(argument) ~= "number" then
        return --Return nothing, so we don't continue in the function.
    else
        return argument^2
    end
end

print(square("hi")) --You can't square "hi".
print(square(5)) --You can square 5 though!