Still not sure why it's required.
It's completely 100% standard these days for game installers to include installers for any underlying APIs (DirectX, Visual C, etc) that are not guaranteed installed on the supported OS. This is because those installers also contains the recommended method of detecting whether a supported version of that API is already installed. If your app needs DX11, it's not a good idea to try and write your own code to detect if DX11, or any other DX version that is DX11 compatible, is installed - instead you just execute the DX11 installer and if it exits without an error, then you're good to go.
Everyone does it. Do you have a Steam account, ever noticed how when you launch a steam game for the first time it runs a bunch of installers, even for things you already have installed? it's the exact same thing going on.
Minimum OS requirements for ED (non-horizons) is Win7SP1, which will always have DX10 installed, so there was no need for a DX installer, but DX11 isn't a guaranteed install on Win7 (it is on Win8) and Horizons needs DX11, so that's why it's now included but wasn't before.