According to the GIRM, the presider’s responsibility extends beyond the homily to the conclusion of the Liturgy of the Word. The references are all to the early edition of the GIRM, but are maintained in the current version of the document:
42. The president is responsible for preparing the faithful for the liturgy of the word on occasion by means of introductions before the readings.  These comments can help the assembled congregation toward a better hearing of the word of God, because they stir up an attitude of faith and good will. He may also carry out this responsibility through others, a deacon, for example, or a commentator. 
43. As he directs the prayer of the faithful and through their introduction and conclusion connects them, if possible, with the day’s readings and the homily, the president leads the faithful toward the liturgy of the Eucharist. 
We’ll get to what the GILM has to say about the role of the commentator a bit later, but feel free to offer comments on any of this on this thread.