Pure speculation here, but it doesn't seem crazy to think that this is in part for use by the Oculus team. While voice commands are a nice to have on current mobile phones - they are likely to be a much bigger deal for head mounted displays.
This acquisition actually makes a lot more sense to me than Facebook buying Oculus. Oculus' ostensible business model was selling hardware to users, which is totally orthogonal to Facebook's business model of selling users to advertisers. IMO there's absolutely no way Facebook bought Oculus just to get the revenue from selling Oculus hardware, but I still don't see what their plan would be for integrating that hardware into their existing business model. I can definitely see how a natural-language API would fit in.
Unlike vomit inducing 'VR' (I own both dev kits) and over broad ideas of 'IoT' - speech and intelligent assistants (ie, siri/cortana/ok google, alexa, jibo, etc) is the future of computing.
If the Oculus team wanted to acquire Wit.ai, they could've acquired Wit themselves (as they recently did with Nimble). Keep in mind Oculus is an independently operating subsidiary within Facebook.