Assuming you are taking about a VFR joiner, rather than an IFR looking to make a visual approach, these are my initial thoughts:
In the case of the former, it gives ATC the confidence that A) The pilot will get to the airfield (ie: is not unsure of position), and B) Be able to position as instructed in the joining instruction, and C) have half a chance of seeing the other traffic you are telling the joiner about.
It has always struck me as a procedure that can lead to problems. If the aircraft does not get the airfield in sight in good time, it runs the risk of blundering into the circuit inappropriately. If the visibility is not brilliant, or for unfamiliar pilots, I personally will try and use an intermediate report such as to 'report with x miles to run' or a prominent ground feature.
An approach controller would not want to put a VFR pilot (who was unsure of position) over to Tower, as the tower controller will often not be in the best position to give guidance to the pilot in order to find the airfield (or an IFR clearance if required). These would be Approach or Radar functions normally.