You name it, and it probably uses it: gas furnace, oil furnace, condenser, heat pump, rooftop unit, chiller, water source heat pump, zone valve, even zoning systems use good old 24vac.
It's also one of the first things you check after line voltage when you're troubleshooting a problem, how many times have you muttered to yourself; "let's see, I have 24 volts between R and C...."
Let me ask you a few questions; is 23 volts ok? How about 22 volts? Is 18 volts enough? Is 27 volts too much?
Let me answer these for you; anything under 24 volts is not enough.
Back in the old days (before electronic controls) control systems used relays, contactors, and pressure switches. If the control voltage was too low the loads (contactors and relays) wouldn't pull in, or they would chatter, or maybe make a loud buzzing sound.
Today things are different. Electronic controls NEED the proper voltage to function properly.
Feed 18vac to a circuit board that requires 24vac and things go downhill quick. If the control operates at all, it's going to act like it downed one too many margaritas (minus the projectile vomiting)
The control may throw random, irrelevant fault codes, it may cycle on and off repeatedly, it could do any number of things....everything except work properly.
Key take away here is; make sure the control voltage is correct. This means it's more important than ever to select the right primary tap on the control transformer. If the system is being supplied 208 volts you NEED to use the 208 tap, NOT the 230 tap.
And remember: just because the control voltage is ok when the system is not running doesn't mean its ok when loads are energized (like compressor contactors) Check the voltage while it's operating also.