You can not sit in front of the TV hoping for your favorite show to pop up. You probably have other stuff to do and a limited time for watching TV. Thats why broadcasters use a teletext form of informatory system called EPG to inform their viewers which channel shows what and on what specific time.
Let`s say you live in the U.K and you are a fan of Sky UK channels. The broadcaster normal provides the service through satellite dishes or cables inside UK. Now what if you are on a job conference abroad and have no access to the satellite broadcasted content. How would you tune to the favorite show? This is possible only through IPTV.