(I'm new to coding)

I'm currently stuck on trying to understand when we need to use parameters inside the

userInput.AddEventListener('keyup',function(**event**) {
let username = event.target.value.toLowerCase();
}

this is the little piece of code however I do not understand when the use of the parameter is needed for it, if anyone can link the appropriate source where I can learn, it will be greatly appreciated.

Just to clarify I do not understand the logic behind function(event) or when is the parameter being used, when is appropriate to use it and so on.