Hi,
Generally, people advice to use CDN for serving static content very fast on your site, that helps you improve your page speed and hence, there it adds value from SEO & UX perspective.
Now, the definition of CDN will help you understand how exactly it can help you improve your page speed. So, here you go:
"A CDN is essentially a network of geographically dispersed servers. Each CDN node (also called Edge Servers) caches the static content of a site like the images, CSS/JS files and other structural components. The majority of an end-user's page load time is spent on retrieving this content, and so it makes sense to provide these "building blocks" of a site in as many server nodes as possible, distributed throughout the world.
When a user requests your site, the node closest in proximity to user will deliver the static content, ensuring the shortest distance for the data to travel (reduced latency), therefore providing the fastest site experience."
Also, check the attached image to understand it well. Hope this helps