Makes them more responsive to your picking. You don't want them too high or they will be muddy and you will hit them while picking. Too low and they won't be responsive enough to the notes that you play. Just play around with the screws that anchor them into your guitar body until you find the right spot.
"For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring." ~Carl Sagan
it gives you more or less output. one small recommendation; keep the bridge pickup lower than the other 1 or 2 it prevents electrical buzz from the other two.