External links are valuable tools for your website to interact with, load resources, or consume services across the web. Some basic examples are the URL of a YouTube video, an image stored on a different server, or a link to a resource provided by a CDN (Content Delivery Network), such as a font or a JavaScript library.
For your website to work according to your needs and users' expectations, it is important that the links pointing to the various external resources operate normally and do not return errors such as timeouts or the typical 404 (Not found).
Doing this type of “URL scanning” manually can be quite a tedious task, especially considering that faults are usually corrected only when we identify them ourselves or when a third party reports them to us.