Is it mandatory to use $wpdb->prefix in custom tables

Sorry if this question is trivial. I’m just beginning to develop plugins in WordPress.

In all the tutorials I found this: while creating the custom tables, $wpdb->prefix is used.

Example:

$table_name = $wpdb->prefix . "liveshoutbox";

My question:

Is it mandatory to use $wpdb->prefix? What happens If I don’t use prefix for my custom tables?