The ideal bearing adjustment is zero play (for the reason I described). But it's impossible to get a perfect adjustment. So you have a choice of erring to the loose or tight side. Preload means that the bearing is slightly over tightened, and within a small range is preferable than too loose by the same amount (
short Tech sheet from Timkin). Be aware that too much preload makes things worse again, so care and judgment are called for.
When I adjust a cup/cone BB I adjust until I can't feel any play, then I tighten a bit more (<1/8 turn), to ensure that I didn't err loose. If all is right I can feel the smooth slight drag of a loaded bearing, but it's still in the range of a loose adjustment. With experience you develop the feel for this. You can also attach the right arm before making the adjustment, which will help you feel any trace of play.
One problem with BB adjustment is that tightening the lockring pulls the left cup back slightly loosening the adjustment. You need to check for this when finished, or compensate for it in setting the adjustment.