Band of brothers?
DW is nice with its high powered heroic space knights fighting for honour and what not, but there is something to be said about playing characters as the real heroes of the imperium.
"Alright men, all we got are 4 lasguns, 2 charge packs, and 1 krak grenade. We've been ordered to take that bunker, so by the Emperor, we're gonna do this!"
If left only in the DH system, you basically would lose alot of resolution, mainly forcing players to be some variation of guardsman. Techpriests and psykers could be present, but scum, arbites, and to a certain extent adepts would be out of place. Clerics would need a new xp table. Battle Sisters would be a tad bit too effective in a combat only setting.
Not to mention that DH would not codify a system for choosing your guardsman's home company (Catachan, Cadian, Tallarn, Mordian, Krieg, Vallhallan, etc.).
The actual fighting of a "war" over say the more surgical style of Deathwatch could also come up. Players could in theory affect troop movements, and engage in campaigns over a greater distance.
Also, a setting where you're put through hell, and surviving to the bitter end is an actual accomplishment is rather cool.
Pretty much this. For some folks, mortal humans appeal more than space marines. I imagine this system will be quite similar to Deathwatch in some respects (building your own regiments, setting out on discrete missions with discrete objectives, etc). Really looking forward to seeing how it's made.