Antennas try to twist and tilt in the wind. That puts a lot of strain on the base where its is bolted to, or embedded in the concrete. If the tower comes to a point at the base, it forms a single point that can flex (albeit just a little bit). The result is less stress and strain at the concrete/steel interface. These tapered bottom pieces are not commonly seen in use by hams due to their high cost (tapered Rohn 25 base section is about $1,400), except when a surplus tower is being used. The tapered base also allows a single insulator to be used when the tower is an active antenna element (quarter wave vertical or etc.). I drive past K3LR every year on the way to and from the Dayton Hamvetion. It's the highlight of the drive.