Application development is a process that consists of many factors. Two main factors are frontend development and backend development. Backend is the place you handle your business logic, and the frontend is the place you will show the data to the end-user (customer). These two things have to connect tightly. The backend provides APIs corresponding with business requirements, and the frontend connects to these APIs to interact with the user.
The developers that work in frontend and backend have to discuss each other, make the decision together about response format for APIs, how the application flow work. If missing discussions, the backend will provide unnecessary things or not the right format API response to the frontend, and the frontend will take more time for parsing and rendering, and so on. Finally, the application will come up with problems such as bad performance, bugs, or even worse.