With seat tube angle, it has a lot to do with your position relative to the cranks. As you come forward you put more weight on your hands.

With head tube angle I'm a bit out of my depth, but I think it has more to do with the style of riding. If you're making a comfortable bike, you don't make the steering aggressive and twitchy, you leave that for "more pain equals more gain" crowd ;-) It'll affect the relative handlebar position... but that's something correctable with the stem length and top tube length... so yeah, I think it has to do more with what you're aiming for steering wise.