The controller is a spectral controller, all it does it put the controller knobs on a digital readout so you don’t need to turn them. You can also create custom spectrum/intensity ratios. IMO unless it’s really a hassle for you to turn the knobs (which really should only be used during initial acclimation) you can skip the controller. After the first few months of turning up the kessils to acclimate your corals, the Kessil fixtures are really set it and forget it, too much tinkering with any LED is just not advisable