as allowances are paid as 'subsistence allowances'. You are not meant to save them. It is like giving a charity worker free food.
I see where you are coming from on that. But surely, the giving of food to a charity worker is done at the time (ie lunch at lunchtime, to be eaten there and then). Our allowances are paid in arrears at the same time as our salary, and we have (presumeably) therefore already spent them in advance.
Your theory would apply more if our allowances were paid to us directly at the time each one was triggered.