Why use “if (!function_exists(‘…”

Checking to see if built in WordPress functions exist before calling them is for backward compatibility which IMHO is not needed.

So if you see if ( function_exists( 'register_nav_menus' ) ) the theme author is supporting versions earlier than 3.0.

You still sometimes see
 if ( function_exists( 'dynamic_sidebar' ) ) 
Why? I couldn’t tell you because dynamic_sidebar was introduced in 2.2.

Another reason to use it is to make your theme or plugin pluggable. A pluggable function is one that can be overridden in a child theme or another plugin.

This is done on the definition not the call and you use the ! operator to make sure it doesn’t already exist before you define it.

if ( ! function_exists( 'my_awesome_function' ) ) {
 * My Awesome function is awesome
 * @param array $args
 * @return array
function my_awesome_function( $args ) {
  //function stuff
  return array();

When this is done a child theme or other plugin can override that function with there own.

source: https://wordpress.stackexchange.com/a/111318