If we just wanted to pass the event object from an event listener to an event handler, this could be done without setting any parameters.

For example:

mobileInput.addEventListener('keydown', validateMobile);

function validateMobile(event) {
	const keyCode = event.which ? event.which : event.keyCode;

	// etc.

But what if, besides the event object, we also want to pass another parameter?

This can be done easily. And with an arrow function - also while keeping the code clean:

mobileInput.addEventListener('keydown', (event) =>
	validateMobile(event, anotherParam)

function validateMobile(event, anotherParam) {
	// Both the event object and anotherParam are available.