Vision tells a robot what an object is and roughly where. Touch tells it precisely what is happening at the single point where contact occurs. Combining them to answer "where on this object am I touching" is harder than it sounds, because the two views have to be aligned in space to a tolerance neither provides on its own.

The mismatch is fundamental rather than technical. A point cloud from a camera describes the object's surface in the camera's frame; a tactile reading describes pressure in the sensor's frame, with no inherent notion of where on the object that is. Fusing them means bridging that gap accurately enough that the answer is useful.

Getting it right matters for manipulation specifically. Knowing you have contact is not enough to decide whether a grip will hold, because whether it holds depends on where on the object you are gripping.