The distinction between substance and accident preserves the fact that what we know is distinct from how we know it, that is, we do not yet enjoy the beatific vision.
The desire to do away with substance as unintelligible (say, in Rahner or Schillebeeckx trying to articulate a teaching on the Eucharist apart from transubstantiation) will collapse the quo and the quod of sensible objects in a way that is only appropriate to the spiritual, and so paradigmatically to pure act. Rather, to locate the Eucharistic change in substance is to place it in that domain of existence that most testifies to our knowledge being in via, i.e. as related to an object that will always be beyond our modes of knowing. Since the Eucharist is the crown of our existence in via, STA’s account of its existence is more appropriate than the contemporary attempts to do away with it on behalf of a supposed physical theory that has done away with substances.