I can tell you what I do with mine
Light timer: Blue, white, refugeum, and under-cabinet lights are all plugged in, on/off with timers. An additional benefit, if the temp exceeds max, main lights go off.
Heater controller: Internal heater thermostat can be set a few degrees higher, giving you a margin of safety on a stuck heater.
Wave controller: Turns my two AC wave pumps on/off as I choose.
Skimmer: Can delay it's startup after a power failure... I've got a high level sensor in the sump, turns off the skimmer. No more flooded skimmer cup.
ATO safety: High level, pH out of range, both disable my ATO system.
Dosing pump safety: Same as ATO, High level, pH out of range, disable the dosing pump.
Alarms: High sump, pH out of range, Temp out of range, loss of communications... I get an email alert, audible alarm.
Data trending: I can see graphs of temp, pH, duration and regularity of refill cycles, electrical amp usage... great as an early indication that something isn't right.
ATO Reservoir refill system: High and Low level sensors and a 110VAC solenoid, let me refill my 5g ATO reservoir automatically. Mechanical float valve for safety.
Water change mode: Disables return pump, reactor pump, skimmer, wave pumps, so that I can do water changes without manually unplugging stuff.
Water change barrel refill: Push a button, my water change barrel refills itself. High level sensor and safety float valve.
Phone based manual control: Want to extend the day for a party? Just switch the lights to ON, override the timer.
I think that's about it, for now. I'm looking at adding a DOS to do automatic water changes... we'll see.