There are a couple reasons. There are those that feel having geographic focused websites is a positive. An example is by US states
.listit_michigan, listit_ohio, etc.
They think having multiple installs in subdomains sharing the same database is a plus using seperate domain names as shown above.
An example would be listit_michigan (Domain name) pointing to a seperate install.
Doing so would allow for each install having entirely different themes and/or modifications, plugins, etc.
People shop the internet but still perfer to buy local. This give them the feeling that they are buying local.
There are more reasons for this and other ways of accomplishing the goal but you get the idea.
Above is not as hard as may sound and being all installs use the same database then maintinance is low.
Tom
