For the primary. It is really useful when your ssd dies (happens more often than you'd think) and you need to keep working because you are operating in a situation where you can't afford to lose the time that would be required to swap disks opening the laptop. We have at least one spare ssd in my office always ready to be swapped in case of emergency.

There are also a lot of cool features on them:

They come with 4 usb ports, 2 ethernet ports + wifi, 1 dvi port, 1 dp, 1 serial port, 1 mpi/profibus port, pcie expansion, dvd unit and bluetooth (which is a given). They also, as per manufacturer warranty, can stand a fall from 1-1.5m without suffering damage.