PFS will ensure the same key will not be generated again, so forcing a new diffie-hellman key exchange. This would ensure if a hacker\criminal was to compromise a private key, they would only be able to access data in transit protected by that key and not any future data, as future data would not be associated with that compromised key.
Note - PFS mode is supported only between Gateways, not between Gateways and remote access clients.
It is not mandatory to use PFS but a extra security layer.
Thanks
Ajay
