Cultivating cannabis indoors makes you responsible for many things that usually Mother Nature is taking care of.
One of such thing is the need to emulate the natural cycle of light and day, causing your artificial sun to rise and set.
Just as the sun can be perfectly replaced by the light bulbs, the rotation of the planet Earth that causes the natural "movement" of the sun can be simulated by using light controls.
The simplest and absolutely necessary level of light control is to be able to switch it on and off, preferably automatically, so that you would not have to keep an eye on the watch constantly.
These simple controller allows you to set the time for the lights to turn on and off, either in 12/12 or 18/6 schedule, depending on your preferred growing technique.
To achieve premium marijuana, you might want to go a step further and try more advanced light controls that allows dimming etc.
For bigger grow rooms it is essential that the light controls are reliable, as they will have to handle high voltage and any sort of breakdown would mean huge lossess.
Commercial growers might get especially interested in control equipment that allows multiple lights (and ventilators) to be connected to a single device.