Moving the saddle back means that your legs and core can hold your upper body weight when you bend over. If your legs are directly under your saddle, then only your arms are holding you up. It's like if you are standing up and bend at the waist - your butt has to moves back or you'd fall on your face. Then it depends on the placement of the bars (height, drop, reach) as to whether your comfy spot is in the drops or the hoods or elsewhere. But you want to be comfortable enough on both to use the hoods on a climb and the drops on a descent.