Is to increase the starting current that can make the tube 
to come up.It contains capacitor.

it to increse starting current , it require for tube lite 
for initial. it contain bi-matalic strip.capacitor is also 
connected for improveing power factor.

The main reason for the starter is to provide the spark 
inside the tube which ionizes the Nion gas inside the tube. 
The Nion's electrones at the last orbit will jumb to the 
next orbit providing light.
the starter contains a capacitor with a very low 
capacitance and has the ability to discharge quickly.

FOR IONIZATION OF GAS IN TUBE LIGHTS, IT REQUIRES HIGH 
VOLTAGE NEAR TO 1100V OR 1200V ACROSS ITS TERMINALS. 
STARTER HELPS CHOKE COIL FOR GENERATING THIS MUCH OF 
VOLTAGE ACROSS THE TUBELIGHT TERMINALS FOR SHORT TIME 
PERIOD SO THAT IONIZATION CAN HAPPEN INSIDE THE TUBELIGHT.
STARTER CONTAINS BIMETLIC CONTACT.AT THE TIME OF STARTING 
IT WILL BE NORMALLY CLOSED & AFTER SOME TIME(SHORT TIME) IT 
WILL OPEN & DUE TO THIS OPENING AS PER LANZ LAW CHOKE COIL 
PRODUCE HIGH VOLTAGE ACROSS THE TUBELIGHT TERMINALS.

Starter is used to help the choke to produce "self
induction". I will explain little briefly.
initially we need 1000v to start the tubelight,so one chohe
was connected series between the tubelight & other end of
the tube light was connected with starter. 
In this starter inside it contains one bimetal strip(open
position)& parallel connected with one capacitor,inside the
starter full of argon gases.
 initially the supply will go through the choke,tube light &
starter.
Already starter inside bimetal strip was in open condition
so supply will not pass it, inside argon gases are present,
so arc will produce between the bimetal strip & it will
close the ciruit(due to heat), so supply will pass throug
it, after then bitmetal stip getting cool and it will open
the contacts.
Immediately the choke gives 1000v due to self induction, so
inside the tube light ionsation was happens & start the light.
why capacitor is used inside the starter? To avoid the Radio
interference.(In our home television or radio is working at
that time if we switch on the tube light little bit noise
will come from the tv or radio, if the capacitor was not
there the noise will be very more this is called radio
interference.)