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");
  }
}

glideMixin(bird);
glideMixin(boat);

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.103 Safari/537.36.

Link to the challenge:
https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/object-oriented-programming/use-a-mixin-to-add-common-behavior-between-unrelated-objects

@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?