48V is the highest standard DC voltage without legal safety issues. Many systems use 12V, 24V, 36V, 48V, 60V, 72V. At/above 60V its no longer "low voltage" so various safety regulations apply. Power over Ethernet uses a nominal 48V for instance.
Note that a multiple of 12V is used due to the availability of 12V lead acid batteries.
Mains supplies producing these voltages are fairly commonly available, I assume that's the original power source,
and that there's a reason to not supply mains cables everywhere within the project. Low voltage wiring can be
much thinner for instance since the insulation doesn't have to be so thick.