It's been my experience that 99% of the people who have desktop boards with dual interfaces do not need them and don't have the supporting network infrastructure to make use of it.
Interface bonding? Requires a switch that can support it to get the most out of it, and one is still limited by the ISP connection.
Failover if one port goes out? That's a possibility, but the chances of a network interface on the board going out are pretty slim.
Printer? Get a switch and plug the printer and the computer into that, job done.
Test network? Sure, but setting up a virtual machine network with something like VMWare, Proxmox or Virtual Box takes less space and power.
Use cases for any system with dual interfaces are small. I use them on my bigger servers as I use multiple LAN segments for traffic separation with QoS and VLAN's, both physical and tagged, via managed switches (HP Procurve if interested). But none of my desktops have dual interfaces as there is no need.
As a matter of fact, I really should split it up further and add another interface to those systems so I can get better redundancy on the backend...
Anyway, as was previously stated, and no offense intended in any way, but if you have to ask, chances are very good that you don't need it.
