One of the best practices in .NET Core and C# is to delete unused and dead code. Unused code is code that is not being used in the application, while dead code is code that is no longer being used in the application. This practice is useful because it helps to reduce the size of the application, which in turn reduces the amount of memory and resources required to run the application. It also makes the codebase easier to maintain and understand, as there is less code to sift through.

Unused and dead code can be identified using code analysis tools such as Visual Studio's Code Analysis or ReSharper. These tools can analyze the codebase and identify code that is not being used or is no longer being used. Once identified, the code can be safely deleted from the codebase. It is important to ensure that the code is not being used before deleting it, as deleting code that is still being used can cause errors and issues in the application.

Here is an example of unused code that can be safely deleted from a C# application:

public void DoSomething()
{
    int x = 1;
    int y = 2;
    int z = x + y;
}

In the above example, the variable z is never used in the application. Therefore, it can be safely deleted from the codebase without affecting the functionality of the application. By deleting unused and dead code, the codebase becomes cleaner and easier to maintain, which ultimately leads to a more efficient and reliable application.