what use or is it useful to have more than one network card in the nas?
Redundancy is one reason : if a network link (the NIC, the cable, the switch facing it) goes down, you have another to stay up.
iSCSI is a protocol that is designed for multipath, so that one will group the capacity of all the links together and provide a higher performance than a single link.
Many jails are meant to be connected by users and often, the NAS itself is for servers only. In that case, you can run your jails from NIC that will be in different network zones than the NAS itself.
But at the end, all of these needs are for enterprise class architecture and solutions. As such, it does not apply to most of home made personal NAS.