There is a new Designer's Journal posted at the FFG Website , which details the whys and hows of 'horde' combat in the upcoming Deathwatch RPG. If you read my synopsis below, you already have a pretty good idea about how this mechanic works and helps gameflow. However, it is a good article, and it is better to hear the real reasons from the horse's mouth rather than my suppositions (although it is interesting how much of what I said they say as well).
Essentially, it makes even a lowly human rebel a threat to Deathwatch Marines (albeit only in great numbers), and makes running mass battles relatively easy and hassle free. Even allies can be run as swarms, such as the Planetary Defense Forces from Final Sanction, or the Stormtroopers from Oblivion's Edge.
The Journal also looks at how certain bonuses affect how the PCs can interact with Hordes, namely the Devastator's ability to deal damage to them more effectively. This is an important factor in the game, and is one way that character creation can have a significant impact on how easy it is for the Kill-Team to fight its way through some missions. In Final Sanction (so far anyway), the Blood Angel Devastator has been instrumental in winning horde combat. Against powerful individual enemies Brother Elyas and Brother Gregor have been the Stars, but that is another story. This article is getting me really excited for the release of this game!
Plus, it has cool artwork. Some of my all time favorite Astartes artwork has been coming out of these news updates. I hope the book delivers more of the same.