Mainframe Computers Mainframe computers are very large and expensive computers often called as big iron machines used by thousands and even more users simultaneously. These are primarily used by big financial organisations(banks and insurance companies), retail giants, corporate and government organizations for supporting critical applications where data security and bulk processing is a key factor. Even though the market is flushed with all sort of computing devices ranging from personal computers, notebooks, tablet computers, but the importance of mainframe computers is still relevant.