Because of the limitations of vacuum tube vs. SS rectifiers, multiple rectifier tubes are sometimes used to pass more current with less "sag" than one. For example, the 1000 series (1000S, 1200S, 2000S, etc.) Sunns use two rectifier tubes. This vacuum tube "sag" can have a desireable "browning" effect on the tone (mostly for guitarists); when situations of high current demand cause some distortion.
In the case of the Boogie, you can choose the tube rectifier if you want the sag effect, or switch to the SS rectifier if you want a cleaner tone, more power (sometimes called "slam" in guitar lingo), and generally tighter and stronger bass response.