This is what you get:
Independent frontend means you can create different shop looks for different devices. At the same time, you can manage the store from the level of one backend. It helps you process orders from different devices using the same for all backend, which is enormous optimization for your business.
Frontend cut off from backend can be created in any technology or framework. It is important in the case of advanced UX designs which usually require specific solutions.
Headless ecommerce stores work faster and are more secure. All this thanks to separation of components that are placed on other servers.
Better security and larger flexibility.