The key to HTC is to efficiently harness the use of all available resources. Years ago, the engineering and scientific community relied on large centralized mainframe and/or big-iron supercomputers to do computational work. A large number of individuals and groups needed to pool their financial resources to afford such a machine. Users had to wait for their turn on the mainframe, and they only had a certain amount of time allocated to them. While this environment was inconvenient for users, it was very efficient, because the mainframe was busy nearly all the time.
As computers became smaller, faster, and cheaper, users moved away from centralized mainframes and purchased personal desktop workstations and PCs. An individual or small group could afford a computing resource that was available whenever they wanted it. The personal computer was usually far slower than the large centralized machine, but it was worthwhile due to its exclusive access. Now, instead of one giant computer for a large institution, there might be hundreds or thousands of personal computers. This is an environment of distributed ownership, where individuals throughout an organization own their own resources. The total computational power of the institution as a whole might rise dramatically as the result of such a change, but because of distributed ownership, individuals could not capitalize on the institutional growth of computing power. And while distributed ownership is more convenient for the users, it is much less efficient. Many personal desktop machines sit idle for very long periods of time while their owners are busy doing other things (such as being away at lunch, in meetings, or at home sleeping).