Let's say you're a penetration tester, you're on-site trying to break into a network, have no Internet connectivity (e.g. because you're in a shielded data center or don't want to raise suspicion) and need to crack some passwords. 5x Skylake CPU + 5x Polaris GPU would be a sufficiently beefy platform for Hashcat, and it would fit into your backpack.

Or let's say you want to quickly set up a Hadoop cluster to crunch some data.

Five machines is certainly small as HPC clusters go, and in reality you'll probably have to cap this to four machines because you need one port to attach to a power socket if you need the cluster for more than 2-3 hours. Nevertheless the 4x 40 GBit/s Thunderbolt ports offer some really interesting possibilities, and I'm not aware of any other portable machine that has this right now.