为什么人们在Wordpress中使用add_action或add_filter的变量函数？

I just was wondering why people do something like :

function world_func() {
   return 'world';
}
add_action('hello',world_func);

Instead of :

add_action('hello',function(){
   return 'world';
});

I see many functions are not re-useable (unique)

Why? What's different? Performance? This is bad ?