Most people raise and lower string height to suit their playing style. Some people like their strings high off of the fretboard, others with their strings close to the fretboard.
Thanks. I knew if was for playing style issues, but I thought it was in relative to pick-up vs. string height. But now that you bring fretboard into it, it makes clear sense. =)
Shellac - changing the string height is to alter the distance between the strings and the fretboard, not the pickups, generally. The higher the strings, the harder to push down, but the less buzzing/clacking etc. you get. So, depending on your playing style, you'll want to adjust the height of the strings in order that they give you the sound you want. If you play lightly, for example, you may well find lower action is preferable, as it makes the strings easier to hold down. But if you've got a more aggressive style, you might prefer higher action so that you can avoid buzzing etc. And if you're playing fretless there's the issue of mwah, to which the action makes a difference (lower action brings out the mwah).
Thanks Moley, that brings more depth into it, for me anyway. I may now have a means for adjustment now that I think about it...