"Proven distribution platform
Steam is the leading digital distribution platform for PC games. We have sold our games through Steam for a while now, and Valve has been a good partner. There may be other platforms that handle certain things in a way we or you would prefer, but they simply do not have the reach, support and status Steam does. Recent upgrades to Steamworks have made it an even more interesting option for us as developer and publisher. These include Delta-patching and more direct administration of our games without a middle-man. Had it not been for that, we would have been much more hesitant to make the decision.
 
Fewer distributed versions
Some of you will remember the huge amount of different distributed versions for Arma 2, and all of the problems associated with it. We needed to spend far too much time on creating and testing all the master copies. Every patch had to be tested for every conceivable combination of distribution, patch and DLC. This drained so many of our resources, that we could not support the game itself as much as we'd have liked. By targeting Steam as our primary distribution, we can take those resources and put them to much better use: making the most splendid game possible.

 
Ready-made functionality
Steamworks offers a library of features which we can fairly quickly hook into Arma 3. Some of them we wanted to develop anyway, but to do it ourselves would again cost valuable resources. External solutions have their own issues, such as the lack of immediate control, but Steamworks saves us needed time. We can take what we want, make adjustments and make sure it all benefits players and mod makers. I am convinced that without this choice, most of the features would not be available at all.
 
Piracy
Think of it what you want, but piracy is affecting us as a business. However, Bohemia Interactive has tried to grow with the evolution of the Internet, rather than to fight it. We have removed intrusive Digital Rights Management (DRM) from our games several months after release, but cannot afford to launch without such technologies. We strongly believe the best anti-piracy comes from offering valuable online services that people are willing to invest in. Supporting that philosophy: we saw we could not deliver in-house solutions on time, but could take Steam's and expand upon them. It will be possible to run Steam in Offline mode, and still play the game (with the exception of online services obviously, including multiplayer itself).
 
Focus on game development
All in all, this means we can as developers do what we like most: game development. And by doing what we like, the results are inevitably better. There is a lot of work that surrounds delivering a game that is anything but creative. Without such work however, we could not survive as a business and keep making games. Please realize that any benefit to us as developers, ultimately should be a benefit to you as player. By going with the chosen approach, we get back into the game.
 

Smoother experience
Our belief is that the previous Arma games have had an unacceptably rough setup experience, involving the installation of various support packages and patches. On Steam, all you do is buy the game, download it, and play. Patching is done automatically, and with the introduction of Delta-patching no longer means downloading gigabytes of data. Whereas a few months ago, you would have to re-download entire add-ons when we moved a single tree in the virtual environment, you now only download the offending few kilobytes. Additionally Steam supports some more advanced tools to participate in experimental versions and testing programs via its beta branches. Players who want the smooth experience can just stick with the default version at all times, others get a sneak-peak glimpse at the future and can participate.
 
Online services
As was eluded to before, Steamworks offers some cool features that we can leverage to enhance the Arma experience where it matters. Internally we did an analysis of which features would make sense in Arma 3, and we even invited you to participate. This resulted in some cool ideas, which are in progress of being implemented. We'll save the exact details for a later update, when we can guarantee these features make it in. One quick and simple thing available to any Steam-powered game is taking screenshots and sharing them. How to best support Dedicated Servers and their admins is another topic we are looking at. To give you two keywords summarizing our priorities: User Generated Content and multiplayer."