For the most part, BB spindle length is determined by the crankset. The 68mm BB shell width is very common and doesn't impact the spindle length (a 108mm spindle in a BB designed for a 73mm shell would result in the chinring sitting the same distance from the bike's centerline as a 108mm spindle in a BB designed for a 68mm shell width).

The (apparently) recommended 108mm BB spindle is sized based on the offset of the crank itself to yeild a "standard" chainline (placing the chainring the correct distance from the bike's centerline). This doesn't mean you couldn't get away with a longer (or possibly shorter) spindle in your, or other, applications. Just that the 108 is (apparently) the norm. Also, chainline is effected by the build of the wheel and where the cog(s) are lined up in relation to the centerline of the bike.

Consider the external bearing BBs like Hollowtech II. There are no options for spindle length. You buy the cups based on BB width and threading. The spindle is integral to the crankset, comes in only one length and is designed to provide the appropriate chainline regardless of the frame.