the eight Chanukah candles, an extra candle is lit called
the Shamash. The purpose of the Shamash is that since the
Chanukah lights are holy, they are not to be used to provide
light [for mundane, everyday uses]. It is therefore arranged
taller or prominently to distinguish it from the other
lights. If one needs light for any purpose, the Shamash is
to be used.
another reason why the custom in many communities is that
women do not engage in any ordinary work for at least the
first half hour that the lights are burning. Women do not
perform chores to make it recognizable that the flames of
the menorah are not to be used for light.