Unfortunately today Subdeacons are often considered nothing more than "glorified altar servers" as one Priest put it.
Besides being assistants to the Deacons, Subdeacons do serve some very useful functions in the Church. Part of their function is to be to guard the doors of the Church and also to maintain the Church in a proper physical condition. They are responsible for teaching the catechumens as well as reading the lessons during the services. When a Bishop presides, the Subdeacons are like his body guards.
Ideally a good Subdeacon knows the rubrics and the various variations that can occur and is prepared for anything that might happen. For example, in many parishes, on the Sunday of the Veneration of the Cross, the clergy and servers leave the altar to venerate the cross at the time of the Trisagion Hymn. A good Subdeacon would be prepared for this and will know exactly what the Priest wants done.
