If a game has DRM on one platform, but not another, that's because the publisher required it. That can be for any number of reasons that don't have anything to do with the platform.
I guess what I'm asking is, what's the benefit to having the installer instead of the game files?
You have to be online to download the installer. The end result is being able to play normally, offline, from portable storage if you choose. Besides a few gigabytes, it seems like a distinction without a difference.