For example, some people think a great wizard's build is Int 16, wis 16, cha 12, dex 12, con 10, str 8 (before racial bonus to int or wis). Why cha 12? So you can get the juice paragon feat spell focus at 11th, when you get cha 13. Dex for initiative (battlefield control works better if you go first).
But there is another juicy feat for wizards, arcane reach, but this requires dex 15. But that requires some hard choices, even with a racial bonus from a dex boost race. Do you stop boosting either int or wis to boost dex by one point? Do you cut initial point allocation to cha, waiting until 21st to get spell focus? Do you cut initial point allocation to int or wis?
Now if one can sack str and con and get two more points, then one can get int 16, wis 16, boost int and wis whenever one can, *and* get arcane reach and spell focus at 11th level (feat retraining is awesome).
I don't know if the game's point-buy vs. feats and other goodies is structured this way for other classes, mind you.