This way if later the gambler makes a wrong decision and if he makes a wrong decision if he loses money then he will still lose part of his profit but in this case his capital will be safe.
What if you only get profit when you already have spent at least half of your capital and now your profit is not even double your original capital and the profit is quite too small to take the capital out and continue gambling?
This can only work if the profit is really high because I think most gamblers would most likely want to keep gambling with 2x the amount of their original capital for even bigger chance of winning bigger money.