What can the second LAN port be used for on a NAS?

So "double the speed" is a bit of misnomer. It is possible to increase the throughput of the NAS by having two ports aggregated together generally this is not to a factor of two.

Also, it is worth noting that link aggregation is not only useful for increasing throughput, but it can also be used to handle link failure, so if a physical port goes down for any reason, you will still have connectivity to the device.

That being said, I don't know about your NAS software specifically, but it is possible then to run segregated services on the secondary port. You could run maintenance services on that NIC, or even the management software from that port. This way you can perform you file transfer functionality on one port, and the misc. services on the other - keeping the port free for file transfers.

On my NAS I like to have a management port, and then the port(s) through which the actually NAS stuff happens.