Intersection tests mainly, I'm sure the reason is buried somewhere in the devlog. At the same time it looked like complex dynamic machines (with pistons, pivots and other parts) were also going to be a priority. It was definitely worth the extra effort, and I still have plans for simpler machines, rope-style bridges with connected parts, and more complex monsters which will no doubt have constraints between their parts.

In addition Box2D has given me super quick spatial tests, which I use for melee, mouse-picking, and ray-casts for the particle system (which is a quick custom built thing).