Simple On/Off Timer : You set the timer to switch on the AC at the specified time. The AC automatically starts at the preset time, and begins to cool the room. However, when you reach the room, it might be a bit too warm or a bit too cool. Basically, you don't get the desired comfort level. Also, you end up wasting a lot of electricity.
On/Off Timer With Advanced Start-up: You set the temperature that you want at the given time. The AC is smart enough to scan the room an hour before the set time. It calculates the time it'll need to reach the desired temperature, and switches on accordingly. So you get the perfect comfort level as soon as you come back, without wasting any electricity.