I have looked this up and the best that I came up with was the bunks are suppose to hold the weight of the boat and the rollers are to keep the keel of the boat from hitting the trailer when unloading the boat. I had wondered the same thing because my boat does not sit on the rollers and it also doesn't have a adjustment to adjust up to the bottom of the boat. I have a tracker pro16 that came on the trailer like that. Maybe others will chime in here with more information if I am wrong.