When you get the refusal letter the reasons given are general and it is hard to know why the VO has denied the visa. The GCMS notes give a more detailed idea of what problems the VO thinks there are with the application. Knowing this, you can look deeper into your history and perhaps find areas you overlooked that may be helpful to include in another application. Examples could be that you did not include ALL your financial information thinking $5000 would be more than enough for a three week holiday. WRONG! The VO wants to see that you are able to earn a decent living at home so providing all your accounts and investment values would be good. Travel History. Travel to other countries for leisure is a big plus. Having a letter from your boss granting your leave and expected return date is also helpful. I hope this answer gives you an idea of GCMS usefulness. Good luck.