Help me understand where neo4j fits

I’ve been trying to work with Neo4j more and more and would like to see how I could do wiring it into a meteor app. I was looking at the neo4j reactivity package but I’m a little confused. Do you keep Mongo and use Neo4j as a service to query mongo data and build relationships or do you use Neo4j to store data as well? Replacing mongodb altogether?

All I’m trying to do is dip my toes into the graph db waters but when I start thinking about it with meteor I’m very quickly lost.