The height of the individual pole pieces helps determine the volume of the individual strings.  For single-note players moving from one string to the next in a solo or a riff, these changes can be quite noticeable.  If the pole piece is especially close to the string, the increase in volume can make an ordinary note sound harsh or abrupt.  Conversely, a low pole piece will make it seem as if the volume has dropped on the notes played on that string, thus sounding like the player is unsure or isn't fretting properly.

The solution is to take a screw driver or Allen wrench (as many pickups --especially by DiMarzio-- tend to have) and dial the piece up or down as required.  This is a bit more precise than the usual approach of simply adjusting the height of the entire pickup to get more output or to match it to the other one(s) in the instrument.

Another factor is the material.  In virtually every case, the pole pieces are made with a ferrous metal that will conduct the magnetic field still farther upward toward the strings.  However, different materials have different properties in this regard and therefore have the potential to yield subtle differences in tone.  Feel free to experiment.

Continuing the line regarding adjusting the pickup volume for individual strings, Austin, TX guitarist Slim Richey found that his Gitane D-hole always produced the aforementioned harsh notes on the B string no matter how low the pole piece was settled into the bobbin.  You notice the missing pole piece?  He simply removed it.  However, because so many people pointed out that it was "missing" (as though these things just fall out all the time), he replaced it with a brass screw with the same threading.  Being a non-ferous metal, the faux pole piece was almost as good as nothing at all.