Dynamic range: in a recording device such as film or a digital camera, it is the difference between the maximum and minimum amount of light that the device can accurately record with detail. The human eye has a greater dynamic range than most digital cameras, or at least the out-put image which, in most cases, is in the .jpeg file format. Using Photoshop, one is often able to able to increase that range and more closely match what your eyes saw – more so with raw image format than jpeg.
Harsh light produces harsh shadows. Facial wrinkles are more pronounced. Who wants to remember grandma with harsh wrinkles. Facial blemishes, like maybe a zit or two, are usually just temporary. With Photoshop these things can be diminished or eliminated. So, why not cheat a little?