The only possible reason for the implementation of this feature I can see is for purely immersive reasons. The possible features of marriage should be as following:
- Sharing properties, mortgages and upkeeps
- Sharing banking features (limited or unlimited, depending on married couple themselves how much finance they want to expose each other with)
- Sharing last names once the marriage is officially registered (creating famous dynasties for famous Empires)
- Inheritance and lineage of dynasty families
- Another small feature to create a viable lawyer role ingame (Lawyers for Corporations, bankings and other affairs will be mandatory I'm assuming)