Also, if fed to a pair of opposite polarity half wave rectifiers and reservoir capacitors, you can get both positive and negative DC rails, which is very convenient for powering many analog circuits, especially ones using OPAMPs.
Nowadays one could use a DC-DC converter for the negative rail, but EMI can be a major concern, and also the rail will come up after the positive rail which may cause problems for some ICs that have strict power sequencing requirements.