What is the advantage of a Private APN?

What is the advantage of a Private APN?
The benefit is that the traffic is secure and never goes on to a public network, staying
completely on GPRS/3G/4G. This lessens complexity in the communications path and is one
less point of failure in the system.
As it is secure, this means that you will not be open to attack external parties, such as
spammers or virus-based activities.