Thanks Rich, you saved me the trouble of explaining it. 
Many (most?) people probably have a dead PC around with a working power supply that will supply at least several hundred watts of pure DC power, so the cost is just a few bucks for the handy binding posts, fuses, and connections to the supply. Putting a rectifier on your transformer will either yield half-wave DC for a diode or full-wave DC that does NOT have a common ground with the AC. Lots of times, you would want filtered DC power, more expense. Finally, this supply puts out a known voltage, having a rectifier and maybe a capacitor for filtering puts out a variable voltage based on load.
On the question of spacing for dual banana plugs, the terminals are NOT correctly spaced to use those, bummer. 
