Modiverse uses a client / server based approach (as most games), meaning that interactions are always handled on the server. However, those of your who have used UE4 know that all user interface actions occur on the client - not on the server.
Therefore you need a way of executing server-side logic when a client interacts with your actor.
Modiverse provides this through a special interface you apply to any prop that you wish interacted with. This interface basically means that when your actor notices a key being pressed, it calls a special function which then (on the server) calls an event on your actor.. Meaning the key press on a clients actor executes your Blueprint on the servers actor.
Of course if you want the action to be client only; then you can do that.. but almost all interactions should be server side to ensure people don't cheat (and almost all UE4 operations must run on the server to work correctly!)