Why use a star or delta connection instead of having a second output
Delta is generally preferred as it gives higher voltage, lower currents and conduction losses. This requires 2 diodes per phase as there is no Neutral return.  ( 6 diodes for 3 phase )

Star (or Y or wye) config is preferred;


when the voltage is too high for the added cost of insulation required but needs only 1 diode per phase and neutral.  
Alternatively, it  is used when when the higher current is desired for maximum power transfer when impedances are closed to being matched.