A couple of thoughtful observations from members at Trinity Reformed Church on Lent and penitential seasons:
Hannah Grieser suggests that there is an analogy between the Lord’s Day liturgy and the church year; the church year is the Lord’s Day writ large. Since we have a penitential moment in our Lord’s Day worship, it seems reasonable for us to have a penitential moment/season as part of the larger rhythm of the church calendar.
Sara Appel notes that there is a lot of penitential literature in the Bible, particularly in the Psalms but also in prophetic books (such as Lamentations). If we don’t have corporate penitential observances, this part of Scripture becomes a guide to private devotion, but we never practice or express penitence corporately, or only do it ad hoc when some disaster strikes.