An unintended side-effect of reading a whole bunch is that one soon gains, not only an ear for how words should be arranged, but an eye for how they should look. A typographical error or incorrectly spelled word will interrupt the flow of one's eyes across the page in much the same way that a six-inch deep axle-eating pothole will interrupt the flow of one's car down the road.
In this modern day and age, when everybody and their grandmother has access to a keyboard and the urge to share their thoughts with the universe, one gets constant reminders of the sad state of literacy in America. Pundits have been droning for years that the new popularity of (email / the dialup BBS / internet fora / blogs: pick one) is triggering a renaissance in literacy. They'll compare it to the days of the late 19th Century, when the predominance of dime novels, written journals, letters, newspapers, and telegrams pretty much forced folks to be able to read in order to keep themselves amused in a world devoid of recorded music, talking movies, and broadcast speech & pictures.
Said pundits must not have access to an internet connection.
Fifty-eleven times a day I find myself beating my forehead bloody against the keyboard, yelling "Who let these illiterate morons out without adult supervision?"
Mauser*Girl is more eloquent on the same topic.