How do you convert GeoJSON data to TopoJSON?
Most likely, because you want a smaller file size. From the TopoJSON docs:
TopoJSON eliminates redundancy, allowing related geometries to be stored efficiently in the same file.... As a result, TopoJSON is substantially more compact than GeoJSON. The above shapefile of U.S. counties is 2.2M as a GeoJSON file, but only 436K as a boundary mesh, a reduction of 80.4% even without simplification.
Also, TopoJSON allows you to incrementally simplify a dataset, if you wish to.