If your website receives heavy traffic and you are hosted across multiple infrastructures across the world or you are using multiple CDNs, then you ought to use a DNS Load Balancer to reroute traffic for better performance.
While there are some large companies who create their own DNS load balancing system, which is complex and tedious to implement and manage, some companies make life easier by opting for ready-made solutions such as those from Cedexis, Dyn, NSONE, Rage4 etc
Most of the above mentioned companies are backed by Anycast networks spread across global locations.
While there are some differences between the feature sets offered by these companies and the number of POPs they own, it is very important to identify the one that works best for your business.