The reason for a double-D is to mashout above 170 so the enzymes stabilize. This is unnecessary if you are going straight from MLT to kettle.
However, the other reason is that boiling the grains give the beer a nice color and add a certain flavor to the beer.
In Process: Mango Beer, Homebrewers Pale Ale
Bottled/Kegged:Spicy Light Rye, Rice-adjunct Pale Ale, Mild Bourbon Porter, Roasty Stout, Basic Light Mead, Bourbon County Stout Clone
Up Next: Berlinerweiss, Chocolate Raspberry Ale, and American IPA