The way I understand it: yes, God forgives me that same instant I repent, whether this instant comes during my sacramental confession in the presence of a priest, or elsewhere.
However, Holy Confession is important because it is a "sacrament" (a Latin term; in the Orthodox language, we say "Holy Mystery"). I don't think we really "understand" rationally, just what is going on during the Holy Confession, excatly because it *is* a Mystery, like Baptism or Eucharist. But we believe that something very important, very essential does happen. It's a part of our "theosis," or "deification," our lifelong path from being a "natural" (or "old") human individual to being a New Creation, a "god by Grace."