The NodeBalancer and all sites behind it must be in the same datacenter. So you cannot serve a Fremont node and a Newark node through the same NodeBalancer.
NodeBalancer also does not serve as a content distribution network (CDN). A CDN tries to serve content from as close to the requester as possible. I have no personal experience with them, but
CloudFlare gets mentioned frequently.
What a NodeBalancer does is balance traffic between multiple nodes serving the same web site*. If a node goes down, the NodeBalancer will stop sending traffic there so your users don't see downtime. If you have a problem with your server getting overloaded or crashing frequently, and tweaking your application's behavior doesn't solve the problem,
and you can set up multiple nodes to serve the same data, a NodeBalancer may be useful.
*Actually, it works for other network applications too, not just the web.