FM,
Usually they write a brief description about the evolution of the relationship including any key points in the relationships (may indicate red flags).
Being able to read the facts/opinions of the officer working on your case can be really helpful if an interview is required. ex. focus on age difference, focus on length of relationship before marriage etc.
I think for most it is just peace of mind that they can follow along with the process and see what is happening with the application.
It is also helpful for confirming receipt of additional documents, identifying any errors made by the IO and knowing when is the next time they will be reviewing the file.
So many people don't receive letters sent to them by the visa office and many have found out about these things through caips/mp assistance.
I don't know if people will write what they learned about themselves in their caips (lol) but many have said it has helped them!!
Sangria
