10 years back we did not have fast computers like today. Today we have super fast computers. Therefore we tend to do multitasks. We use several applications at the same time. Imagine you are writing something on Word whilst Photoshop is minimized and you want to restore Photoshop. What you would usually do is going to the Taskbar and click on Photoshop.
Instead of that hold Ctrl down and press Tab key and release. Then a rectangular box will appear on your screen and Photoshop will popup instantly.