Yes, CDN is worth paying for.

Here is what CDN does
A CDN is a service that will download large pieces of your website, such as images, and save it locally in worldwide global data centers.

Letís say you host your website on a server in Washington D.C. If someone in China wanted to access your site, the web pages have to be downloaded from your servers in Washington D.C to the userís computer in China. This could take a few seconds. A CDN helps resolve this issue.

CloudFlare is one such good CDN.