Pre gastric byp surgery diet?

I am going to have gastric by-p surgery and I have been given a pre-operative diet that I am supposed to be on for two weeks before surgery. It's 1000 cal, all sugar free, high protein, no solid food. I'm just wondering why. Are they trying to have me lose as much weight as possible before surgery or is maybe it a process of making sure the stomach is completely empty? I guess I'm just wondering how important it is to comply with such a strict diet