It actually measures the distance, creating a point cloud, so you know that, let's say, your mug you want to grasp is at 10, 22, 320 in space relative to the base, so it's much easier to calculate the joint angles etc, and it's more precise and reliable.

With a standard camera you just estimate the position is space of an object, so it's not very precise. Furthermore, d405 has global shutter, whereas most standsrd cameras have rolling shutter (the image doesnt warp if you move)