A Motivation for Distributed Systems
Mainframes historically represent a centralized computing model that focuses on efficiency, local scalability and reliability. While this model is very effective, it is also very expensive and out of the reach of many companies with the cost of memory, storage units, processing cores, etc. reaching into the millions of dollars. As a result, many see Distributed Systems as a less expensive way of achieving and exceeding the raw computing power that typical mainframe configurations represent. That being said distributed systems do not preclude mainframes.
A distributed system consists of any number of computer configurations such as mainframes, minicomputers, personal computers, etc. The ultimate goal of Distributed Systems is to network a group of computers to work as a single system.
from "Reactive Application Development"
Quoted on Mon Mar 9th, 2015