You can manage the license on their dashboard. If I remember correctly, you can also enter a license already used and have the app disable the license in the other location.
Parallels is cheaper than VMware for sure, but the company is focused on macOS and maintained with frequent updates. They need to pay developers.
You can say that is not really useful for a MacPro5,1, but how can something be cheaper than free?
Both VMware Fusion and Workstation are now free for personal use.