Why to create a let variable for mixin?

why in this challenge the example create a let variable to store the mixin?

this is my answer and it worked well when i did without create a variable to store it

let bird = {
  name: "Donald",
  numLegs: 2

let boat = {
  name: "Warrior",
  type: "race-boat"

function glideMixin (obj) {
  obj.glide = function () {
    console.log("im gliding");


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Link to the challenge:

@credoangel Out of curiosity, when you first created this post, did you get a warning about possible unformatted code?

yes it did, english is not my first lenguage so i didnt get what does that mean why? that is important?