Why Use -48 VDC Sources?

The only telecommunication equipment that are standardized to use the -48V DC are the telephone equipment. Providing sufficient and safe power for continues signaling and tone generations. An industry standards for equipment manufacturers.

I am not sure what you mean exactly...but.

I worked in telecommunications and a pulsed 50 v dc was standard for line operable...the buzzing you hear when you pick up the phone.

Probably just a matter of a good voltage with enough current to do the business of making a phone call......i.e. 2 volts would be not good and also 300 volts would be not good.

It is just safe"ish" and recognisable to the system down not the best quality cable.

Just to add to your reply....

The most obvious reason for the 48 / 50 volt option is its a nice multiple of lead acid batteries i.e. 4 x 12 volt batteries.

The telephone network is designed to be fail safe and is expected to work when mains power is not available.