First Web App with JS, used a bit of animation and transition

I started studying Basic JS about a couple of months ago and this is the first time I'm using it. I gotta say that it's totally different from what I've studied. LOL

Situation: I removed the event listener and the script is still working perfectly. document.addEventListener("DOMContentLoaded", function (e) {...}); Question: What would be the advantage of leaving the event listener on, if any?

Please feel free to comment on the transitions, animation, and/or HTML structure that I used, as well.

Thank you!