The needles meter the fuel allowed into the fuel circuit by the main jet. All needles have a taper to them (first pic). You can adjust how rich/lean the main circuit is by changing either the needles, jets, or shimming the needles. Shimming/changing the needles is the easiest way to richen/lean the mains because they are easiest to get to.
For grins, here's a couple of pics showing idle/low & wide open throttle positions:
Here's a graph showing the interaction (clip = shim, sorta):
