[coolkil]

You use it in an environment where you need an extreme level of data integrity and availability. Think banks, airports, government, etc.

Due to io offloading to co-processors and a whole range of supporting cpu types you can set up a system which can handle thousands of transaction per second. Most of the application that are used on mainframe's are databases or message que's

Recently there is an increased interest in running Linux on mainframe's something which can be done since early 2000 you get the benefits of high available and secure hardware and the relative ease of Management of Linux. Another benefit is that you don't really need to train personnel in more exotic operating systems like z/OS.