In the physical world the difference between the version of your OS that can MPP across a cluster of servers and workstations and one that can't is stark. Among virtual machines, where workloads are already distributed and resources assigned logically rather than physically, the line is fuzzier.
If you had a workload prepped to run as a series of MPP processing units, you could probably run it against Azure without the HPC addition, as long as the number of work units wasn't too large.
With the HPC, that's not a worry.
It's still not a real high-performance computer, but it does give much more granular control to users over the number of VMs their workloads are spread across, the resources they use and the amount of time even compute-intensive jobs spend inside the blue cloud rather than chugging around on the cluster inside your own data center.