|There could be several reasons. I suspect the biggest is, in case you need to have the server connected to multiple subnets.|
I know we have many servers at work with 4 or more network interfaces in them (usually with the addition of a 4 interface card) because they connect to multiple subnets.
If your server is only going to be connected to one network then you'd have a spare in case the first one fried. At least, you could switch over to it faster than if you had to down the server, open it, add another NIC, then close, rack and then fire it up again.