USA Social Security question.

[Kiwiberry]

My daughter gets Social Security benefits. She is supposed to be receiving a large retroactive back payment. Her regular monthly payments can be spent on food clothing you name it as long as it's for my child. However I had to sign a form to agree to only spend her back payment on medical or legal fees. I'm just a bit confused because it's money that she was owed each month which would fall under her regular monthly payments. I'm wondering since it's the same money just in a larger sum why I can't spend it on her everyday needs? Are there any other moms out there with any experience with this? I also had to open up a second dedicated bank account just for the back payments because the one she has for her regular monthly payments was apparently not good enough. It just honestly makes no sense to me.