Eye Dilation And Thinning Retinas

If you've been to an optometrist lately, you probably had to get your eyes dilated. At least, the option was presented to you, whether or not you took your doctor up on it. But is it really necessary to determine the health of your eye? And what did it mean when your optometrist told you your retina might be thinning and to "keep an eye on that"? Is Digital Imaging as Good as Eye Dilation?