It gives you High Availability.
For storage for example, with iser on the client (initiator) side if you create 2 iser interfaces, one on ib0 and one on ib1:
iscsiadm --mode iface
default tcp,<empty>,<empty>,<empty>,<empty>
iser iser,<empty>,<empty>,<empty>,<empty>
iser-0 iser,<empty>,<empty>,ib0,<empty>
iser-1 iser,<empty>,<empty>,ib1,<empty>
and then login to the same target over each interface
scsiadm -m node -p 192.168.1.10 -T iser-target --login -I iser-0
scsiadm -m node -p 192.168.1.10 -T iser-target --login -I iser-1
you will get two block devices, e.g. /dev/sdf and /dev/sdg. lsscsi should list them.
Then you put them in multipath and you get /dev/mapper/mpathN devices which you can fdisk/mkfs or create logical volumes like on any other block devices. multipath will route IO over iser-0 and iser-1. The default algorithm is round-robin.
On the server (target) side 2 interfaces will also give you HA. But it's also important that the target side network does not become a bottleneck. If you have 2x40Gb ports in the target server and 80 clients then each client will be limited to 1Gbps if they all do IO simultaneously. So you should go for 4 or more ports in the target.