question on HOH Vol 2 - multiple inflection points

[DMC0627]

I am having trouble with one of the problems. Pages 223-227 discuss a situation with pocket eights utg. He advises raising to 400 (or folding) if playing the hand to tie yourself to the pot and give odds to call an all in from another player. Why not just go all in then? I do not understand this, if you are willing to call an all in, isn't it better to push all in unless you are slowplaying a monster?

I went through his vol 1 without many problems, but am struggling with the inflection points. I also seem to do well full table, but once its down to 5 or 6 players, I have trouble adjusting. I bubble frequently in sit and go's, and am trying to play a better shorthanded game. Any advice or suggestions as far as what book (besides HOH) to help with this is appreciated.