About declaring parameters in 'Rock, Paper, Scissors' exercise

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#1

The hint for step 12 (create the function playGame()) is
const playGame = () => { const userChoice = getUserChoice('scissors'); const computerChoice = getComputerChoice();

I wondered why we don’t have to write const playGame = (userChoice, computerChoice) =>. Is it because we will declare the parameters later inside the function block?

Then if we choose to declare first, I figured out we can write the above code block as below:
const playGame = (userChoice, computerChoice) => { userChoice = getUserChoice('scissors'); computerChoice = getComputerChoice();

This post is to ask if my understanding is correct!

#2

If you make a function add that takes two values and return the sum - what parameters would that have and why does it need parameters? Does your function need parameters for that same reason?
How does a function communicate with the rest of the world? Where do values go in, where do they go out?