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When discussing code reusability, we delve into one of the fundamental advantages of functions in programming. The concept refers to the practice of writing code that can be used again without modification in different situations or projects, much like the parts of a car engine that can be employed in multiple car models. Imagine you've written a function to calculate the sum of two numbers, similar to the `add_numbers` function from our original exercise:

With this function at your disposal, you can reuse it in various parts of your code without the need to rewrite the logic for adding numbers each time. This not only saves time but also reduces the chances of errors, as the function has likely been tested and debugged thoroughly.

Benefits of Code Reusability

• Efficiency: Save time and resources by reusing existing, reliable code.
• Maintenance: Updating a single function can improve all sections of the program that use that function.
• Readability: Reusable code tends to be organized, which makes it easier to understand and manage.

In our example, by reusing `add_numbers` within another function (`multiply_and_add_by_2`), we exemplify the power of reusable code fragments in a program.

The concept of modularity in programming corresponds to the design principle that software should be divided into separate parts, or 'modules', each handling a single aspect of the functionality. These modules should be interchangeable and able to function independently. In the context of functions, modularity refers to having distinct functions that perform specific tasks which can be combined to achieve more complex operations.

Take the functions presented in our exercise: `add_numbers` and `multiply_and_add_by_2`. Each function represents a module with a unique job. By keeping the addition logic separate from the multiplication logic, we increase our program's flexibility and make the overall structure more manageable.

Advantages of Modularity

• Improves collaboration: Different team members can work on separate modules simultaneously.
• Simplifies debugging: With modular code, pinpointing and fixing errors becomes a less daunting task.
• Enables scalability: You can more easily add new features without overhauling the entire codebase.

Our example's separate functions demonstrate modularity – they can be developed, tested, and maintained independently while contributing to the program's greater whole.

The term function calls refers to the process of invoking a function in a program so that it executes the task it is designed to perform. When a function is called, the program flow jumps to that function, the code within the function block is executed, and control is returned to the point where it was called once the function work is complete.

A function call can include passing arguments to the function, which are the input values that the function uses to perform its operations. As seen in the exercise, the `multiply_and_add_by_2` function calls the `add_numbers` function and passes the result of a multiplication along with the number 2 as arguments, like this:
result = add_numbers(multiplication_result, 2)
After the `add_numbers` function completes its execution, it returns the sum back to the `multiply_and_add_by_2` function.

Key Takeaways of Function Calls

• Sequencing: Function calls dictate the order in which various parts of the code execute.
• Decomposition: Complex tasks can be broken down into simpler, individual function calls.
• Flexibility: Functions can be called multiple times and in various contexts within the same codebase or across different programs.

The dynamic of calling one function from another, as shown in the exercise, illustrates the encapsulation of functionality, an integral aspect of structured programming.