If all the priest does is "talk" after the readings, we're in big trouble! Regrettably, there will always be clergy who are less gifted or improperly prepared for the task of proclamation. The result is yammering, which leads to boredom, impatience, and questions like this one from the assembly, I'm afraid. So let's get the terms right. The priest (or deacon) is not supposed to talk, give a speech, teach, or even offer a sermon at this time in the Mass. What we're supposed to hear is a homily.
So the question we're really asking is: What's a homily and why do we have one? The word is rooted in the ancient word for "conversation." A homily isn't merely a conversation between the preacher and us but between the scripture we've just heard and the world we live in. Think of it as a call-and-response, like others we hear at Mass. For example, when the priest says, "The Lord be with you!" we reply, "and also with you!" If the Lord isn't with somebody when the call goes out, the conversation stops right there. The call must find its response or there's no real communication.
In the same way the scripture passages proclaimed at Mass (not simply "read," if your local lectors are doing their job) are best understood as a call awaiting our response. God's word is alive, the Bible says. But not alive on the page—it's alive in those who hear the word of God and keep it! It takes some reflection, of course, for us to appreciate how to live out the word we hear proclaimed each week. So that's where the homilist comes in. It's the job of the one who gives the homily to build a bridge between the ancient word of God and the modern world in which we live and move and have our being.
While a sermon can be a moral teaching on any issue of the preacher's choosing, the homilist must remain anchored to the readings, which are preselected for each week of the church year in a book called the lectionary. That can be challenging, but it does ensure that we hear from a wide range of scripture passages each year—not simply the preacher's particular themes of interest or expertise. The homiletic method means both homilist and assembly must grow together as we participate in the conversation God is having with us today.
Nehemiah 8:1-12; Luke 4:16-21; 11:27-28; Acts 17:28; Hebrew 4:12
Fulfilled in Your Hearing: The Homily in the Sunday Assembly from the U.S. Catholic bishops
Once Upon a Gospel: Inspiring Homilies and Insightful Reflections by William J. Bausch (Twenty-Third Publications, 2008)
The Practical Preacher: Handy Hints for Hesitant Homilists by Paul Edwards (Liturgical Press, 1994)