While mortal sins require knowledge venial sins do not.
One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.
Also, as Christians we should desire to do the right thing and to know what it is. While the act may not have been a sin - or a venial sin in our love for God we should desire to do his will. Jesus calls us to be perfect. Attaining perfection requires knowing what faults we have so that we can work on them.
A forgotten sin is forgiven during confession. Remembering them is not required for forgiveness. However, it is good to remember all your sins while in confession for the same reason that a sin should not be deliberately withheld (See quote). However I am not sure that it is independant of attaining perfection.
Christ's faithful strive to confess all the sins that they can remember, they undoubtedly place all of them before the divine mercy for pardon. But those who fail to do so and knowingly withhold some, place nothing before the divine goodness for remission through the mediation of the priest, "for if the sick person is too ashamed to show his wound to the doctor, the medicine cannot heal what it does not know."