We go -48 -> 48 -> 12 -> 3v3,1v8 etc etc. If you went 48 straight to POL voltages then you would have horrific converter performance.

Likely as a basis for converting to other useful DC voltages.

Well if it's negative 48V the electricty flows out of your circuit and back to the grid, so you need to make it positive to have the electricity come in.