CDN is Content delivery network. If your clients are across a vast area then CDN usage is a must.
With Usage of CDN images and other static resources will be fetched from CDN servers which are located at multiple locations.
This means that visitor to server distance will be pretty less and probably no hops. Which in turn will mean in lower latency time.
If your Magento website page had 20 images and other 40 static resources then the overall cumulative latency can run to 2-3 seconds or even more.
The second benefit of CDN is that it reduces server load as instead of requesting your website server requests are sent to CDN server.
A proper selection of CDN provider is a must and one should check server location while taking decision.
are some of the known names that should be used.
By using above you will be able to reduce your Field time by approx average 2 seconds across your visitors which in turn will mean in higher conversion.
The ROI on use of CDN is pretty high and it should be a must.
For more details on Magento optimization check this article :