No Pain No Gain?

I am wondering if anybody has some useful advice as to the true function of gain settings and their sonic effects other than volume control. I have a ML 32 which has the possibility set set gain for each input at 0/+6/+12/+18db and an EMMlabs CDSA-SE cd player that has a high/low gain option (I believe +14/+18db). What would be the optimal setting? Or is it totally based on personal taste, or is this functionality only really for matching volume levels for several differing sources?

Any helpful comments on this topic would be greatly appreciated.

Henry