The introductions are meant to prepare the listener for the readings that are read. It is appropriate to prepare those listening especially on those time the readings are quite heavy. The Collects usually prepare in part for the Gospel proclamation, but not for the other readings.
We generally read the short introduction right before the Proclamation of the Word. As they are available they will be posted at this blog. All are free to use them as one might wish. Grace is free! Shalom!