Understanding NLB between WFE and App Servers

There are two WFEs with high availability. I asked someone if you are using NLB between them to load balance and he said yes.

Then he said we also have two app servers with high availability. I asked if you are also using NLB for them? He said no and he further said servers are not load balanced but only service applications. (what ever that means)

So what I gathered from it:

1. You need NLB to load balance WFEs
2. But you don't need NLB to load balance app servers because it is done by SharePoint itself.

Is my understanding correct?