Here are just a few things you can do with your skimmer, return pump and ATO when you connect them to your APEX.
Skimmer:
You can turn off your skimmer when your return pump goes off, so it doesn't keep running and overflowing when the water level in the sump rises. You can have it react to float switches, you can turn it off during feeding (Feed Mode A, B, C, D), you can schedule it to run only at certain times. You can turn it off when dosing. You can have it wait X amount of time after a power outage before it turns on. You can turn it off if the pump is pulling too many or too few Watts/Amps (2016 APEX). You can setup email/text notifications if any number of things happen.
Return Pump, you can turn if off during feed modes, have it react to a float switch in your overflow, and shut down and not flood your house if the return lines get clogged, If it is an apex ready DC pump, you can run it at different % at different times, you can use one of your feed modes as a maintenance mode and shut it down for cleaning, etc. You can schedule it to shut off at certain times of the day/ certain days of the week. You can put an outlet on your Fusion/Local and power cycle the Return pump from anywhere in the world. You can turn it off if the pump is pulling too many or too few Watts/Amps (2016 APEX). You can setup Email/Text notifications if any number of things happen.
ATO:
I don't know what your ATO setup is, but mine is a Tom's Aqualifter that reacts to a float switch, which keeps the sump water level consistent to within 1/8th of an inch. It also has a 2nd float which is a backup in case the 1st switch fails. You can also disable your topoff and be notified by email or text if the Salinity drops outside the range you set.
There are many many reasons to connect those things to your APEX. Many more than I have listed here.