Sometimes people ask, "how do you decide what to preach about?"
As you might notice from below, frequently my daily homilies are more Scripture-focused--that is, because we have continuous readings at daily Mass during Ordinary Time, I find it is a good time to do some "Bible study."
I like to focus on the first reading (though not exclusively), because it alternates each year, while the Gospel (at daily Mass) remains the same; the Gospels, therefore, tend to be more familiar. Also, frequently our Gospel is a homily from our Incarnate Lord himself, and as I sometimes say to the congregation, how can I add to that homily?!
I'm not sure, but I think with daily Mass-participants, being a little more "studious" about Scripture is more likely to bear fruit: these are folks who have their own copies of Magnificat or a daily missal, and who are more likely to read the Bible on their own.
Of course, none of this is an iron law. A priest-friend of mine generally focuses on the saint of the day, which I do from time to time. Sometimes, I focus more on the Mass itself, as I did somewhat in today's homily. And, yes, sometimes I am more focused on doctrine or spirituality, which is where my Sunday homilies end up.
People hear all sorts of things about what the priest (or deacon) is supposed to preach about. The truth is, the preacher has very wide latitude (in no order): the Scriptures, the liturgy itself, the season, some doctrine of the faith, a saint, even topical events -- all insofar as it relates to "the Faith."
The goals, broadly, are "kerygmatic" and "catechetical" i.e., the call to faith/conversion and instruction; and you can get into lenthy, arcane discussions, if you really want to, about which is more important, etc.
One more thing: while preaching is required on Sunday, it is not -- although encouraged -- at daily Mass. I generally do preach, though sometimes I just invite people to reflect on what they heard, and I sit down for a few minutes. I don't know what people prefer; I go both ways in my own mind about it.