The pre-programmed holds (as opposed to unplanned holds) exist so that if there were any unexpected temporary hitches in the launch sequence, there is still time to make the launch window. Imagine for example that you have a 1 minute launch window, and a slower than expected fuel pump means that it takes 5 minutes longer to top off the fuel tanks. Well, you just missed your entire launch. Stand everyone down, and put the spacecraft back into standby before trying again tomorrow. If you have a planned hold, you simply reduce the planned hold time, and you can still hit the launch window.
The hold is also used as a sync point between all of the different activities that have to be completed at certain points in the launch preparation. For example, at the t-4 hold for the Curiousity launch, they needed to pass the go/no-go from the entire team. If one team had been running late up till that point it wasn't a problem, the other processes just entered the hold earlier. Once the last hold has been cleared, all of the remaining processes have to stay synchronised.
The hold is also used as a sync point between all of the different activities that have to be completed at certain points in the launch preparation. For example, at the t-4 hold for the Curiousity launch, they needed to pass the go/no-go from the entire team. If one team had been running late up till that point it wasn't a problem, the other processes just entered the hold earlier. Once the last hold has been cleared, all of the remaining processes have to stay synchronised.