So, first off, why ArcGIS Server? Well, ESRI is the industry leader in mapping and geospatial technology (and gives us many opportunities to put “geo” in front of existing technologies to make them our own). ESRI is a dominant player in federal, state, and local map-making and location-related data. Since many government groups want to make their data available for internal as well as public use, they can use ArcGIS Server as a platform to share that data for those who need it.
I highly recommend the book, so go get it.