@Bambos
pfSense itself never needs outbound NAT rules. It's rather the outside world, who needs it.
The point is to enable the outside world to communicate with your local devices, which probably resides inside a private subnet.
If the outside world has no route to your subnet pointing to your (VPN) interface IP, you need to masquerade the source IP on outgoing packets with the interface IP with an outbound NAT rule.
If you have a site-to-site VPN the remote site has usually a route for your local subnets. So there is no rule needed then.