Having an entire site physically non-functional is rare in medium to large organizations because standard best practices would require that the server room be located at least on the second floor, and sufficient circuit breakers and other measures be in place to prevent physical damage caused by natural disaster such as flooding.

However, for any kind of business, smaller sites can really be just a sales office and contain a DC that has multiple functions, such as print and file server. Once this office and the single DC get damaged, the entire site is out. If the office was then in an area where WAN connections are already slow, and of bad quality, having the entire office use the WAN for authentication can cause serious strain on the network resources. Of course, the WAN link could also get damaged in the event of a disaster, as the networking equipment is usually kept in the same room.

Recovering an entire site is a very straightforward process in itself, and is closely...