24 VDC voltage is very popular as the control voltage of choice in Europe. Some automakers have adopted 24 VDC in the USA as a standard for control applications since only one maintenance person is initially needed to fix a machine problem.
While an electrician and a machine repairman are required with 120 VAC machine problems, low voltage DC requires only one common repairman to fix most down-time problems.
With the newer Arc-flash rules in place, 24 VDC has become the dominant voltage for most control circuits. The 115 VAC circuit is soon to be gone (or at least in many applications).