Question: We normally use abstract function/Interfaces in our projects. Why it is really needed? Why can't we just go for Business logic Layer, Data Access Layer and Presentation Layer only

**Function in Presentation Layer:**

```
abc();

```

---

**Function in Business Logic Layer:**

```
 public void abc()
    {
      //Preparing the list
    }

```

---

**Function in Data Access Layer:**

```
public abstract void abc();

```

---

**Function in Data Access SQLServer Layer:**

```
 public override void abc()
    {
       //Connection with database
    }

```

---

**Question is: Why is the Data Access Layer required ?**

Answer: 
The abstract class or interface is not really a separate layer - it should be part of your business logic layer and it defines the interface that the actual data access layer (SQL data repository, for example) needs to implement to provide the data access service to your business layer.

Without this interface your business layer would be directly dependent on the SQL layer, while the interface removes this dependency: You put the abstract class or the interface into the business logic layer. Then the SQL layer (a separate assembly, for example) implements the abstract class/interface. This way the SQL layer is dependent on the business layer, not the other way around. 

The result is a flexible app with an independent business layer that can work with multiple data repositories - all it needs is a layer that implements the interface the business layer defines. And it is not really only about data repositories - your business layer shouldn't be dependent on the context (asp.net vs. console app vs. service etc.), it shouldn't be dependent on the user interface classes, modules interfacing with your business app, etc.
I think you are talking about Facade layer. 

It is an **optional layer** which will simplify the functions of Business Layer. Let's imagine, you have a ProductManager and CategoryManager and you want to do a particular action which involves using both (for example, get me top 5 products in all categories) then you could use a facade layer that uses ProductManager and CategoryManager.

It is inspired by [Facade Pattern](http://en.wikipedia.org/wiki/Facade_pattern).