Global content distribution service networks improve site performance. The two factors they address are network latency and application scalability.
Scalability of a site can be increased by distributing the request load over a greater number of servers. In local load balancing, these servers are colocated in a single location.
This is adequate where the site serves a single region and only requires greater throughput. However, the single location approach will suffer from the effects of network latency where the client is distant from the selected physical location.
Content distribution networks address network latency by moving the content closer to the client using geographically distributed servers. The key to influencing the client to use the server with the lowest network latency for that client is the deployment of geolocation aware dns services.
While geodns is not widely available outside of bundled CDN services, it is a standard feature of edgedirector.