What is the function of SNAC?
SNAC service is Network access control module of SEP.
Since SEP has all modules(AV,NTP,PTP, ADC & SNAC) under one agent you will see the srevice present.If you have license of the SNAC then the service can be enabled, else it will be in stop mode.
How it work?
How to Install Symantec Network Access Control when SEPM is already installed and Running
http://www.symantec.com/connect/articles/how-install-symantec-network-access-control-when-sepm-already-installed-and-running
How to Install Symantec Network Access Control
https://www-secure.symantec.com/connect/downloads/how-install-symantec-network-access-control
Is it required different license?
Yes