 When you fast, you're weakening the flesh again. Yeah, you may repent of that sin. Yeah, you may receive the forgiveness of God, but it's also a good idea to not only receive the forgiveness of God, to not only turn from that sin, but to get the flesh back in a weakened state so that habit doesn't return. So think about how it takes months, weeks, days to fall into the ways of the flesh. Well, when you fast, it brings you back to the spiritual state so that you're not in the fleshly state that caused the problem in the first place.