The TV Network Doesn't Matter

All my life I've seen commercials by TV networks... saying something like "ABC... must see TV". They all do them. But I've always wondered why they bother to do that. If a show I like is on TV, I watch it. I don't care what station it is on. Also, I have no preconceived notion as to whether some network would likely carry new shows that I would like better than those on other stations. Why do they waste time and money doing this?