It's to cover the cost of temporary accommodation and also food.
Now days the you get the cost of the accommcdation and few £ on top i think about £10. Any money you make on top is due to working lots of OT . Not a thing i go for but have mates who do it and they tell me 100% the only real money they make is due to doing lots of OT ie 5+ hrs a day.
As for getting £500 a week just for working away dream on.
these comments are my own and in no way represent any company that i may or may not be linked too.