Soul Reapers cleansed souls that turned to hollows, either naturally or ones the invaded from Hueco Mundo. Soul Society organized all of this. Quincy’s did the same thing basically.
So what did the Vasto Lordes do before Aizen showed up and gave them a purpose and plan? Did they just sit in Hueco Mundo being powerful for no reason? Did Barragan and others rule the hollows and if so to what end?