Sometimes I use manual exposure mode, especially for flash photography. When I review the shots, I often find it difficult to figure out what was the ambient exposure. Did I underexpose by 1 stop? 0.7 stops? 1.3 stops? Etc. Without knowing the amount of ambient underexposure, it's difficult for me to replicate the ambient/flash balance with certainty.
One solution is to take a baseline shot in a semi-auto mode (P, A, or S). Nothing fancy - the baseline shot doesn't even have to be sharp or clear or well-composed. I just switch to program mode and take a shot without flash. That will give you a pretty good idea of the amount of ambient, from which you can later recall the amount of ambient underexposure in the actual shot by comparing the exposure with the baseline.
An alternative baseline method is to take a shot in manual exposure with the same settings but without flash. I think that's useful (to see the ambient contribution to the shot) but taking the program mode shot tells you the amount of underexposure in more numerical terms.