Why is USPS boxes free?

Can you explain to me why the United States Postal Service (USPS) offers free boxes for shipping purposes? I'm curious about the rationale behind this policy, as it seems to be a unique offering in the logistics and shipping industry. Are there any specific benefits or incentives for both customers and the USPS in providing these boxes at no cost? Additionally, I'm wondering if there are any limitations or conditions attached to the use of these free boxes, such as size restrictions or certain types of items that cannot be shipped in them.