Mainframe computers are big and they exist in their own climate controlled room (or facility). Not because the operators like it at 70 degrees Fahrenheit, but the computers need to be stored in cooler rooms because they can run very hot. They are sometimes referred to as “Big Iron”. That name comes from the olden days (1950-1980s) when these big boys came in big, heavy, steel and iron cases and cabinets.
Mainframe computers are data kings. They are high-end, commercial machinery. They are very expensive and very labor intensive to keep running properly. When there is a problem, it usually takes a small team of programmers to iron out the problem. (Hence, also why they are called Big Iron). They can plow through an incredible amount of bulk data that needs to be adjusted, realigned and calculated, and then spit right back to you very quickly (on paper, screen, tape,or disk. If you need 1/28 of all of the electric bills for the city of Columbus, OH, recorded, calculated, printed out and mailed, a mainframe is what you want. It is exactly the type of computer that would be used to calculate that data and is currently used in Columbus. Columbus is the 16th biggest city in the USA – that is a lot of bills. It takes a lot of programming.
On the other hand, they are not good at small projects. So, do not ask the mainframe to check your e-mails or do a grocery list. They are not set up for this triviality and it would take you a lot longer to program them for these tasks than for you to just do them yourself. The US Census uses mainframes. The census has over over 300 million people to tabulate and it does not take 10 years to do it. It takes more like 10 months to add it all up, proofread it, calculate it, size it and print it, and correlate to make the reports. If the job is big, give it to a mainframe computer to complete.