am i ment to leave the poilte light/flame running?

im am trying to understand thins water heater. im am only young and dont know if im using it right or am i just been blonde!!!

i have a britony water heater. am i ment to leave the poilte light running all the time?? or am i ment to light it ever time i wish to use the water? leaving it on all the time does this waste lots of gas?

Please help me!!!