The negative oltage generator on board the lcd is used to supply a negative bias for the contrast. The one side of the contrast pot is connected to Vdd the other side to Vee (the negative voltage generator) and the wiper of the pot is connected to Vo.
Checkout the POWERTIP website for this info.
All extended temperature range lcd's need a negative voltage to adjust the contrast.
If you understand german you have a look in "Halbleiter-Schaltungstechnik" Autor: Tietze/Schenk. They give some applications for temperatur compensation!