When laundering with soap (just that, not detergent), with hard or even moderate water it was often advised to add packaged water softener to wash and first rinse. This would bind hard water minerals to keep scum and other muck away from laundry.
Hopefully by second rinse much of soap had been already removed so chances of scum forming was less.
Detergents aren't bothered by hard water (one simply adjusts dosage according to local water hardness), so in theory adding water softener to rinse isn't required. See discussion in another thread elsewhere on forum about importance of rinsing when doing laundry.
When using detergents making water overly soft (as in using too much packaged water softener, especially phosphates) can cause issues. Again the surfactants used in detergents aren't as bothered by hard water minerals compared to soap. Thus rinsing shouldn't be as difficult because unlike pure soaps introduction of fresh hard water won't cause detergents to bind to wash.
If one is attempting to strip laundry, then phosphates can be used in both wash and rinse, but then you don't use detergent or soap.