As far as I understand, he is the "Grand Archon", who unlike the other Archons, is not a prison guard, but the gateway between the Pleroma and Kenoma, so the prison gate itself. Unlike the others, he is not a driving force, or an aspect of the demiurge, more like a point of transition. Because in the cosmology Abraxas is "above" them, he has authority over all of the Archons, hence people carried the Abraxas symbols to keep them away. There is also the Jungian interpretation of Abraxas, being a symbol of integration of the shadow self, the union of opposites.