The adjustment is there to allow you to balance string outputs. If it sounds OK to start with then don't bother, but if one string sounds particularly weak, then you have the option of raising the pole pieces up on that string, and if that's still not enough, down on the other strings to get the best balance you can.

On a P-bass pickup, you can adjust the height of each end of the split pickups to achieve the same result.

Of course, any major imbalance is quite likely to be down to a duff string or the way the saddles are set up rather than the pickups.

As Waz says, the Entwhistles don't appear to be set up to allow a huge adjustment range, so be careful if you try it, Wind upwards, rather than downwards if possible.