Adds an FAQ entry why the IMU is required in 3D (#328)

docs/source/faq.rst

@@ -34,3 +34,20 @@ measurements, for matching. Since there are two VLP-16s, 160 UDP packets is
 enough for roughly 2 revolutions, one per VLP-16.
 
 __ https://github.com/googlecartographer/cartographer_ros/blob/master/cartographer_ros/configuration_files/backpack_3d.lua
+
+Why is IMU data required for 3D SLAM, but not for 2D?
+-----------------------------------------------------
+
+In 2D, Cartographer supports running the correlative scan matcher, which is normally used for finding loop closure constraints, for local SLAM.
+It is computationally expensive but can often render the incorporation of odometry or IMU data unnecessary.
+2D also has the benefit of assuming a flat world, i.e. up is implicitly defined.
+
+In 3D, an IMU is required mainly for measuring gravity.
+Gravity is an attractive quantity to measure since it does not drift and is a very strong signal and typically comprises most of any measured accelerations.
+Gravity is needed for two reasons:
+
+1. There are no assumptions about the world in 3D.
+To properly world align the resulting trajectory and map, gravity is used to define the z-direction.
+
+2. Roll and pitch can be derived quite well from IMU readings once the direction of gravity has been established.
+This saves work for the scan matcher by reducing the search window in these dimensions.