Really, what the app amounts to is a "guide" to confession - how to examine your conscience, make a confession, etc., none of which are a bad thing.
How come we have to "go" to confession? How come I need to go see a priest, detail my sins, and ask for forgiveness? Why can't I just tell God I'm sorry?
You can (and should!) tell God you're sorry. Right now. Every day. However, that isn't enough. In the Gospel of John, Jesus clearly gives his Apostles the ability to remove sins from others. This is clearly the practice of the early Church, as demonstrated by the Church Fathers.
Now, you can get into a verse-slinging, historical hair-raising argument if you wish, but I propose that the Sacrament of Confession has a very human basis: we need it. We need to sit down, really recollect our sins, think about what we've done and haven't done, and then say that out loud. We need to hear the priest, acting in persona Christi, say "you're forgiven". Just like all the other sacraments, confession needs a human face, human hands and a human heart to make it real for us. We don't baptize via the internet or marry by phone. We are body AND soul, and the body part can't be left out.
As Christians, we believe we are Christ's hands and feet here and now. And there's no app for that.