What happens if Data relationship is not created in Marketing cloud?

Question

I am new to Marketing cloud. We have integrated Sales cloud with Marketing cloud using V5 MC Connector. While exploring current system which is setup by an other team, I could see there are lot of Data Relationship built under Data relationship tab.

From documentation, i understand that it is used to connect two different data extension. For Eg : I could see a data relationship built between Account DE(Synchronized DE which bring Account object from Salesforce) with Contact DE(Synchronized DE which bring contact object from Salesforce).

1. Why is this mapping done?
2. what happens if the data relationship is not created
3. How different is this from contact builder where a contact key is mapped against Data extension?

Answer 1

After going through documentation and by practically working, i found the answers

1. The data relationship is necessary for joining two different data extension.
2. If data relationship is not created, then creating a filter using these two data extension is not possible.
3. Contact key mapped against a Data stream dataextension is a standard functionality that comes with V5 integration.