As for the reason why it started, didn't it have to do with the networks wanting their own individual listing in the TV guide (generic, not just the magazine) back in the days when they all had lists instead of grids? Rather than having Channels 6, 10, 13, and 17 all beginning their late shows at 11:30 PM, three of them would be in a listing at 11:30, and one would get its own listing at 11:35, in theory making it stand out more.
I recall reading that somewhere, although I'm sure a bunch of people will point out how horribly wrong this is. And of course now that every listings service is grid-based, starting at a different time won't do anything to make your program stand out. Never mind, of course, that all that late shows now begin at 11:35. :-)
(OT: Which online TV guide do you folks use? I liked Titan TV, except that they keep insisting on adding random channels to my favorites lineup, and there's no convenient way to get rid of them; instead you have to go to the clunky all channels interface.)