WordPress uses an HTTP class to make outbound requests. These requests are mainly for plugin, themes and core updates; pull news/rss content; and make third party API requests.
There may be many different reasons for disabling outbound requests, security is the primary one. A WP instance with disabled outbound requests is more secure. Many institutions have intranets or secure networks that do not allow external requests. If the outbound requests are not disabled within WP, then WP admin console will not function properly due to timeouts.
The main disadvantage of blocking outboud requests is blocking third party APIs or any external requests. There are many plugins that will not function if you do so.
If you want to disable WP from making any outbound requests add these settings in wp-config.php:
define('WP_HTTP_BLOCK_EXTERNAL', true); // block external requests
define('DISABLE_WP_CRON', 'true'); // block WP cron