Beyond the time management issue, though, I find two additional advantages to re-preaching old sermons. The first is what it gives the congregation. The younger Me who wrote that sermon however long ago was dealing with different issues, interested in different topics, approached things differently from how the present Me does. That means my congregation is receiving the gospel on a wider bandwidth, not confined to what might be called my current obsessions.
The second advantage is what re-preaching gives to me. That younger Me is sufficiently removed from my current interests and approach that in preparing to re-preach that old sermon, it is almost as if I am having the gospel preached to me. And that challenges me and comforts me in important ways. I almost never get to receive the ministry of preaching -- I'm always on the giving side -- so I listen carefully to what God has to say to me through my younger self.