Contactors make sure your lights turn on and off reliably. HID lights require a boost of energy to initiate the arc. This boost is drawn through the timers. If you use a timer without a contactor, it can lead to timer failure. This will then cause inconsistencies in the light cycle which is not something you want to happen, especially during the flowering stage where night cycles need to be maintained consistently. Contactors allow you to switch lights on and off without overloading your timer. When selecting a contractor, make sure it is going to be suitable for running the amount of lights you have.