Documenting the process will enable your business to ensure all the correct steps are taken in the right order, that you haven't missed anything and also allows you to review it from time to time and improve the process.
It is also extremely important from a training perspective to provide information to new staff to help them understand your processes and get up to speed quicker with the right information.
You can also use this policy to educate your client/s on the process so they understand when and how to make a complaint.