[matrix2003]

As mentioned in another comment, it’s also close to the safety limit for low voltage systems.

IMO solar pioneered (in recent history) 48V DC systems, which is an easy multiple of 12V to stay below the 50V “high voltage” safety threshold.

It allowed people to use smaller gauge wire and chain together multiple 12V batteries that are readily available.

[bryanlarsen]

Telephony has been using -48V as a line voltage for a very long time, probably > 100 years.

[matrix2003]

Is that for power deliver, or signaling?

[p_l]

-48VDC was standard for powering telephone equipment, and is also common standard for direct current datacenter connections.

Signaling on POTS easily hit over 100V, btw.

[amanaplanacanal]

I remember getting zapped when the phone rang when I was a teenager back in the 70's. Good times.

[Kirby64]

Both. It powers old school telephones.