In reply to SafeNath:
If your belayer is stood away from the foot of the wall, and you fall off, there could be some horizontal pull on the bit of gear you've placed - i.e the rope could pull on the bit of gear and tug it towards the belayer, and possibly pull it out (this has happened to me once).
If in this sitaution you had put in a low runner beneath the main bit of gear, the rope would exert a downward pull on the higher bit of gear you've placed, as the lower bit should take the horizontal pull (if placed well). This should stop the higher bit of gear being ripped out.
Cant think of any other reason to have low runners, unless its multi pitch of course, where a low runner above a stance will stop a factor 2 fall from skocking the belay anchor and rope..