Why use layout before loading information?

This is called Preload overlay and, as they said in the other answers, it is a user-friendly practice, showing a layout and a warning that the page is loading, for example.

This can be done with Jquery / Css / Javascript, using a code similar to the one below.

Jquery

  jQuery(function ($) {
  var target = $('#target');

  $('.toggle-loading').click(function () {
    if (target.hasClass('loading')) {
      target.loadingOverlay('remove');
    } else {
      target.loadingOverlay();
    };
  });
});

HTML

<div id="target" class="loading">
  <div class="loading-overlay">
    <p class="loading-spinner">
      <span class="loading-icon"></span>
      <span class="loading-text">loading</span>
    </p>
  </div>
</div>  

CSS

    @font-face {
  font-family: "demo";
  src: url('fonts/icons.woff') format("woff"), url('fonts/icons.ttf') format("truetype");
}

@keyframes loadingStart {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}
@keyframes loading {
  0% {
    transform: rotate(0deg);
  }
  50% {
    transform: rotate(180deg);
  }
  100% {
    transform: rotate(360deg);
  }
}

.loading {
  position: relative;
  pointer-events: none;
}

#css-input:checked ~ .loading .loading-overlay {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  -webkit-animation: loadingStart 3s 300ms linear 1 both;
  -moz-animation: loadingStart 3s 300ms linear 1 both;
  -o-animation: loadingStart 3s 300ms linear 1 both;
  animation: loadingStart 3s 300ms linear 1 both;
  background: rgba(255, 255, 255, 0.5);
  text-align: center;
}
#css-input:checked ~ .loading .loading-text {
  font-size: 0.875rem;
  line-height: 1.3125rem;
  text-shadow: white 0 0 1em, white 0 0 0.5em, white 0 0 0.25em;
  position: relative;
  display: block;
  text-transform: uppercase;
  font-weight: bold;
}
#css-input:checked ~ .loading .loading-text:after {
  content: "...";
}
#css-input:checked ~ .loading .loading-spinner {
  position: absolute;
  top: 50%;
  bottom: 0;
  left: 0;
  right: 0;
  margin: -3.9375rem auto 0;
  color: #1a1d1d;
  text-align: center;
}
#css-input:checked ~ .loading .loading-icon {
  font-size: 4.8125rem;
  line-height: 5.25rem;
  text-shadow: rgba(255, 255, 255, 0.75) 0 0 0.5em;
  -webkit-animation: loading 1s steps(4) infinite;
  -moz-animation: loading 1s steps(4) infinite;
  -o-animation: loading 1s steps(4) infinite;
  animation: loading 1s steps(4) infinite;
  display: block;
  vertical-align: middle;
}
#css-input:checked ~ .loading .loading-icon:before {
  vertical-align: middle;
  content: "\e000";
  font-family: "demo";
}

This code example I removed from here .

If you would like to use a plugin to do this, you have this

When a web application has to load a lot of information, it is better to present the user with such a screen and load the data "underneath" rather than displaying a simple blank page with no information. This is a user-friendly issue.

To do something like this there are many ways, with a simpler combination of Javascript / jQuery / AJAX with CSS.

When it's an application that has a lot of data to load, instead of adopting a blank page with loading, it already displays the layout template without the information and a warning that the information is being loaded. This is what happens there in the case of Slack, which you exemplified with the photo.

Front-end technologies, such as HTML / CSS and Javascript, should be used in this case.