The sermon is basically open-mic time for the priest. They generally offer commentary on the gospel, how it relates with the other readings/psalms of that mass, current events and so on.

With the sermon being what it is, it’s one of the big ways in which a priest can influence their congregation. The priest doesn’t select the gospel, readings, psalms, etc. it’s all published in a book called a Missal, and does not change very often, so they can’t cherry-pick certain passages etc. Given as well that priests are regarded as (in effect) the “messenger from God”, parishioners will often alter their thoughts and behaviours based on these sermons.

Now, that being said, the priest is that church’s “store manager” - hired and appointed by the diocese and is responsible for more than just saying mass, they run the church. Think of what a store manager would do - HR tasks, maintenance, advertising, and so on - is all the head priest’s responsibility. If there was no priest to make a sermon, that church would have been shut down.