The important difference is that when you tax only the land and not the value of the improvements on top, it doesn't discourage improving the land to its highest and best use the way that property taxes do.
For example, downtown properties with surface parking lots on them (or similarly underdeveloped uses, like self-storage warehouses) ought to pay the same tax as the skyscrapers next door. That's how you make it stop being profitable to build shitty surface parking lots and self-storage warehouses on prime real estate.
Ditto for building McMansions on 1-acre lots instead of bungalows on 1/9-acre lots (or better yet, townhouses or small apartment buildings) in neighborhoods just outside of downtown.
It's a slightly different take on property taxes. Basically, the idea that all land is taxed at a fixed rate per acre no matter what is built on top. It's also known as Georgism