| The practice began before the advent of steel strings as a way to make the gut strings brighter which I suppose is desireable for solo playing, and it helps the bass stand out from the rest of the ensemble.
Solo tuning is still widely practiced even since the invention of steel strings. I don't quite understand why. Modern solo steel strings are designed to come to pitch at the same tension as standard/orchestral strings, yet somehow they still sound brighter. | 
