Importance of confirmation letter?

I have never heard of a confirmation letter. If you continue to be employed by the company, that is the confirmation that you passed probation. The appointment letter states "Your salary may be re-evaluated on confirmation" which may be an internal process, it doesn't mean you will get a letter or any documentation in confirmation of your appointment. Also it does not give any indiciation as to whether your salary would rise or be lowered on completion of probation - that would depend on your performance, attendance, general attitude, and quality of your work. If you want your salary re-evaluated, talk to HR about the procedure required.