Many have speculated it kept the president in the sitting position making him a better target.
The President's personal physician, Dr. George Burkley, drafted the death certificate on November 23, 1963.
It stated the back wound was "at about the level of the third thoracic vertebra."
One of the autopsists, Pierre A. Finck, was questioned under oath February 24, 1969, in State of Louisiana v Clay L. Shaw:
Q Can you give me approximately how far this probe went?
A The first fraction of an inch.
Also, re-considering that the throat wound was an entry wound versus an exit wound, it might also explain why there was no spray of blood and tissue from the front of Kennedy's neck as the bullet exited.
QED the wound was one of entry.