It's designed to let the television viewer know when the player is there.
There was a time when that computer generated yellow line was not shown......it was up to the viewer to survey the field's yard markers in conjunction with the 1st down markers on the sideline in order to know where exactly the ball had to cross to make a first down......
Now, with this new technology, I can be well into my 2nd 6 pack and STILL have somewhat of a clue.