still a bit confused about using genarateHTML function as an argument without argument

It's not about the argument, it's about the parentheses ().

A function can either be declared (the moment where you define function). Which usually looks like this:

function myFunc () {
   return "Hello World"
}

or you can call a function which usually looks like this

myFunc()

or, you can refer to a function, which looks like this

myFunc

The last option is not calling the function, it's simply mentioning it. It's basically pointing to the function.

Why would this be useful you may think?

It's useful because in javascript, you are allowed to use functions like normal variables. This means you can pass a function as an argument to another function. Or a function can return a function.

This will allow you to do some cool things like this

function calculate(operation, firstNumber, secondNumber) {
  return operation(firstNumber, secondNumber)
}

function add(firstNumber, secondNumber) {
  return firstNumber + secondNumber
}

function subtract(firstNumber, secondNumber) {
  return firstNumber - secondNumber
}

console.log(calculate(add, 3, 6)) // 9

note how we passed a reference to the function add without calling it, we called it inside the calculate function.