A leftover from machines of five to ten years ago, the turbo switch still remains on many cases, even though it really serves no purpose any more. In the early days of the PC, there was only IBM, and there were only a handful of different speeds a PC could run at. Early software was written by programmers who believed they were writing it to run on a machine of a specific speed. When newer, faster machines would come out, some of this software (especially games) would stop working properly because it would run too fast. Turning off the "turbo" function of the PC (which meant anything that made it run faster than an IBM of a particular era) would make the machine run slower so this software would work. In essence, it was a "compatibility mode" feature, to slow down the machine for older software.
Now, there are dozens of different combinations of processor types and speeds. Software cannot rely on knowing what the speed of the machine is, so most programs use speed-detection algorithms to determine how fast the machine is. The turbo button no longer serves any useful purpose, and in fact on many motherboards there either isn't anywhere to connect it, or there is a place but the motherboard does nothing when you press the button. The best use for this button is to never touch it, or use it for some other purpose. Some older machines will still slow down when the button is pressed, and if you press it by accident your machine will lose performance; it can be surprisingly hard to track down the problem, since it seems that the front of the machine is the last place anyone appears to notice anything. You can correct this problem if you find yourself doing this frequently.
Fortunately, the turbo button has all but disappeared from modern system cases, especially newer NLX, ATX-family, and WTX form factor systems.
