Two reasons Dee, first is to handle any ph swing and the second is gassing off any CO2 and/or Chlorine. Proper aging requires that an airstone is added to the water change barrow to gas off any CO2 and/or Chlorine. If you read around you will see a stable ph is recommended. Subjecting discus to a ph swing during a water change is a stressor to the fish and can affect them health wise over time.
I personally go thru between 80 to 160 gallons per day depending on which tank is due for a water change. I am running two 75s and two 40s. The two 40s have growouts and those fish get two water changes a day (yes, they are BB). The 75s have adult wilds which get a 50% every other day. My aging time is about four hours to handle the ph swing. My ph out of tap is 7.4 and in four hours raises to 8.2. However I age longer to get the temp in the aging barrow up to the tank temp.
hths,
Pat