Why to create a let variable for mixin?

JavaScript
1

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);

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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

3

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