Ultrasonic Range Finder problem

Hello,

Here i have attached the zip of few programming files of Ultrasonic rangefinder. I have used ATmega8 on board controller.
I want to know the purpose of usage of MCU in Ultrasonic Rangefinder as we are already interfacing it with the microcontroller. Why the MCU is used here and do I need to program the on board ATmega8?

If I need to program ATmega8, then which file among these are need to be loaded in MCU? Why these files are used here and what is the purpose of rest of the files?

it would be appreciated if one can explain the working of this Ultrasonic Rangefinder. It is same as HC-SR04 Ultrasonic Rangefinder module. The MCU is specified here which is ATmega8. LM324 and BAV99 is used here. So it would be appreciated if anyone can help to understand the working of this module. Please note, I know, why and where the Ultrasonic sensor is used. I know about the ultrasonic sensor fundamentals like soundwave, reflection, transmission and all but here I want to know about the working of the circuit that is easily available on various websites.

Please help me to solve my doubts.

Thanks.
ChiragView attachment src.zip