Quick Javascript question regarding Vue custom 'scroll' directive from the VueJS cookbook

I am learning VueJS from the VueJS documentation. In particular, I am in the topic on custom directives at the moment. I reference the docs if anyone needs more information..

VueJS Creating Custom Scroll Directives – #base example

With the custom-directive code block below:

 Vue.directive('scroll', {
  inserted: function (el, binding) {
    let f = function (evt) {
      if (binding.value(evt, el)) {
        window.removeEventListener('scroll', f)
    window.addEventListener('scroll', f)

…I am having trouble wrapping my head with respect to the (binding.value(evt, el)) expression for the IF statement within the f function. Could someone please explain to me what the expression means? I’m thinking that the IF block is there to improve performance by removing the event listener between each ‘scroll’ event? I however never heard that it was required to remove the event listener. I would appreciate if someone could provide a more concise explanation on what it does and why its done that way.

Thank you very much for your time!

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