why use css variables code example

Example 1: css variables

:root {
  --background-color: #333;
  --text-color: #fff;
}
body {
  background-color: var(--background-color);
  color: var(--text-color);
}

Example 2: css variables

/* "css variables" */
/* not for ie but good for Edge and other browser :D */

/*
 By declaring a custom property on the :root pseudo-class
 and using it where needed throughout the document
*/
:root {
  --primary-bg: #8a2be2;
  --btn-font-size: 18px;
  --btn-padding: 10px 15px;
}

.btn-primary {
  background-color: var(--primary);
  font-size: var(--btn-font-size);
  padding: var(--btn-padding);
  color: #E2E2E2;
}