Hosts can be single user or multiuser. A host requesting a service. A host can be both a client and a server at the same time. Any task carried out by a server on behalf of a client. Distribution of computer resources is key.
Computers are large and expensive.
Shuttle data files from one site to another - cheaper than buying computers for each site. 1990 ARPA no longer funds Internet. All over the world. Large number are from the U.S. and Western Europe. In the past, academic and military research.