This is on 4.12 Preview 2 (Launcher Version: 4.12.0-2962397+++UE4+Release-4.12), I tried to make a floating AI move around in 3D space using “Move Directly Towards” (which, according to the Tooltip, moves directly to the target without regard to any navigation system).
In practise, the Node fires, it takes some time to complete, but the Pawn does not move at all. As soon as a NavMesh is placed, the movement works correctly (which might be tricky in a freely floating space, but can be achieved with hidden planes).
To reproduce, create a new project, then create a new BP derived from either Pawn (which then needs a FloatingMovementComponent) or DefaultPawn (to get the component for free). Disable “Enable Gravity”, set Movement Capabilities to either “Can Swim” or “Can Fly”, then add a simple Behavior Tree that first picks a random point in space, then calls MoveDirectlyTowards on the selected point. “Project Goal Location” could be disabled, and for my case neither “Tracking Moving Goal” nor “Allow Partial Path” are required.
As is, this won’t do anything, the pawn won’t move. Place a NavMesh, and it suddenly works.