From their website "KML is a reasonably simple syntax that you can work with in any text editor. For very simple documents, it is easy enough to do just that. However, if most of your spatial data resides in traditional GIS formats (shapefiles, coverages, ArcSDE) it becomes very tedious to translate these values by hand. Likewise, most of this data probably resides in projections other then what Google Earth expects. Finally, ESRI's ArcMap is an excellent cartographic authoring environment that many in the GIS community are used to working with. Arc2Earth was built to remedy these problems and to make it as easy as possible to translate all of the information inside an MXD file into KML. While not perfect, Arc2Earth will translate as much information as possible including Labeling, Renderers, Graphics and Layouts."