One fairly common use case for the RSVP Pro plugin is for event management and wedding companies to offer the plugin in a white label fashion. An easy way to do this is by enabling a WordPress Network (aka a multi-site instance), however, until recently the user experience was not the best in this case. Depending on how you wanted to look at it you were installing it on a single instance, however, you would get an error that the license was invalid. The multi-site license is a way to fix this so the error goes away and it continues to work better in a network.
Now when you install the pro version of the plugin and network activate it you can specify the license in the network admin section. As long as you have a multi-site license everything will validate correctly and no more invalid license messages will appear.
What happens if you install the pro plugin on just one blog in a network with a single-site license? It will operate just like you would have if you had installed it on a standalone WordPress instance as it should.
The other use case for a multi-site license is if you are a consulting shop that is going to end up using it on more than nine sites and so instead of doing multiple purchases you can just purchase it once and use the same license key for all of the sites.