Pre-Op Diet vs. No Pre-Op Diet

Im curious to get input as to why some surgeons dont have patients do a pre-op diet? What are the benefits of doing the pre-op diet? What do you feel about it?
Any input is helpful

The surgeon that performed my sleeve actually does it both ways.  I was told that is all depends on how much "belly fat" you have and also how fatty your liver is.  I personally did not have to do a pre-op diet.  I would say that the people in my support group are split 50/50 on who did and didn't have to be on one. 
I think it all comes down to your safety in the operating room.  The surgeon wants to eliminate/reduce the chances of something going wrong during your surgery.

Thank you for all your information. It was very helpful and now I understand!  the difference.
Thank you again!