Why use more than one network port

a few, mainly redundancy.

There are three benefits.

1. It can be used to provide redundancy
2. it can be used to connect the NAS to multiple local networks
3. it can increase transfer speeds when you have multiple clients accessing the NAS simultaneously

For most home users, the transfer speed increase isn't very important, since there generally isn't enough simultaneous access to saturate a single gigabit link very often.

Note that for (1) and (3) you need to use link aggregation.  Most of the link aggregation modes require switch support, and you do need to coordinate the mode on the NAS with the mode on the switch.  The ALB and TLB modes don't require special switch support, though ALB sometimes will misbehave.