1st step in the data base design is data modeling. Data modeling involves conceptual data modeling and physical data modeling. E-R modeling is widely using to tool for conceptual data modeling.
A data model describes the structure of a database. Structure of data means describing the entities involved in the data base and their attributes, relationships among the entities, keys on entities , constraints etc… That is, the E-R model gives the clear picture about the database.
E-R modeling is a communication tool. It helps the data base designer to develop a good data base that meets the end users and application programmers. Especially, E-R modeling gives a pictorial representation of the database. Thus, the database designer can understand the data base description easily.
Therefore, unless the data base is modeled well, database cannot be developed well.