You’re missing the point that praying for souls in purgatory greatly benefits us living here on earth too. First, we are praying.
Second, we are doing a spiritual work of mercy.
Third, assuming we’re in a state of grace we are accumulating merit for our act of praying for the holy souls.
Fourth, if the soul we are praying for is a loved one, or the group of souls includes loved ones, we are renewing our connection to them and helping our own peace of mind.