As people have said, this is likely for being ground truth. But let’s clarify this a bit. Currently Tesla are relying on neural networks, these are trained using pairs of input-output data. Input is camera images, output is BEV(birds eye view) grids etc. These output labels are created by humans aided by computer vision, ie the point cloud that karpathy showed in Autonomy day.
My guess is that Tesla is using this Lidar data to create a test set to see how well the vision system is working and to add tests to their unit test set. After training on data from camera from normal cars, they make sure that the neural network, using camera data from the modified Y will output what the Lidar in the modified Y is saying it should predict.
Maybe it will turn out that the Lidar is measuring the distance to the vehicle in front to be 100m, while the neural network is predicting 90m. Then they will know this and can try to find ways to improve the neural network, for example by gathering more data, cleaning up old data, tweaking the network, finding a bug in the code etc. Maybe a few months later it now predicts 95m and they can say that their algorithm is getting better. So the ground truth is not used for training, it is used to test how good the trained system is.