Digital sensors do not respond to light in the same way that film does. Film responds in a similar fashion to our eyes. The human eye has built in compression (we see in a non-linear fashion) which allows our senses to function over a large range of stimuli. According to Bruce Fraser "If you expose digital the way you expose film, you run twin dangers of failing to exploit the camera's dynamic range, and creating exposures whose shadows are noisier than they need to be."*
There may be a temptation to underexpose images as to not blow the highlights which can run the risk of introducing noise in the midtones and shadows. If you underexpose and make adjustments during raw conversion this can also exaggerate noise and introduce posterization. This is again where exposing to the right can be useful and in Camera Raw using the correct settings on the Exposurer slider and highlight recovery you can recover as much as one stop of highlight detail.
I found this online pdf very informative and helpful.
Our cameras perform "in-camera" processing so that the image is presented in a totally different fashion from it's original state, which left in its unprocessed state would be very dark.The camera's processor applies a gamma correction curve to give the image a normal appearance. Exercise 5 has been formulated to give greater understanding to the gamma correction curve.
Linear - A relationship where doubling the intensity of light produces double the response, as in digital images. The human eye does not respond to light in a linear fashion.**
Nonlinear - A relationship where a change in stimulus does not always produce a corresponding change in response. For example, if the light in the room is doubled, the room is not perceived as being twice as bright.**
* pdf "Raw Capture, Linear Gamma and Exposure" was written by Bruce Fraser. It was adapted from his book Real World Camera Raw, published by Peachpit Press, in August, 2004.
**refer to Lowrie C K (2007 p248/249) Digital Rebel XTi/400D Digital field Guide
**Lowrie Charlotte K (2007) Digital Rebel XTi/400D Digital field Guide City: Idianapolis Wiley Publishing Inc