It is a filler and it wil stretch their gut out. We feed it dry to everything in the showbarn out of convienence(we've tried wet, but just prefer it dry). It helps with adding belly on one and solidifies manure. The heifers get more of it then the steers used to b/c I want them full without eating as much feed. If we are pushing one, we give just enough to keep the stomach in tack and try to just go at it with feed. As said, over time it can make them want to eat more by stretching the stomach, but if you are in a hurry and you have a good eater with enough capacity I see no reason to feed it other than to help with the manure. Its basically all about where the calf is in comparison to where it needs to be. You have one thats too fat, put in a lot of BP and oats and less feed to hold. One thats running behind needs feed. One on track needs a touch to keep everything in balance. A tight gutted one may benefit from the fill.
I definately reccommend feeding BP at shows because of its filling values. Feed it dry and let them drink and the BP swells and will fill them out. You want to introduce it to them a fair bit before the show to eliminate upsets from changing the ration. Just what seems to to work for us.