One of the stranger aspects of the American Healthcare system is how it relies on insurance companies to provide care. This came about due to the way that medical care was historically viewed.
In the past, medical care was viewed as something that was rarely required, and families hoped they did not require it. Medical care was expensive and thus reserved for emergencies. Minor care like a physical was most likely to be encountered in a group care scenario like school checkups or verification of health for a job or military service.
In this context, classic insurance companies began to enter the market to insure against unexpected events. These insurance companies recognized that they needed to have a relationship with those providing the care (not uncommon for insurance companies to control the vendors used for repairs or replacements) and built large networks of physicians. The original two networks were under the Blue Cross and Blue Shield names. These networks would eventually merge and are today known under the combined name of Blue Cross Blue Shield.
This system of paying to resolve a specific medical issue is known as pay-for-service.
To support this system, the industry standardized on a paper form known as the UB-04 form. The US Government adopted this form for Medicare and Medicaid under the name CMS 1450.
These forms would eventually start getting submitted electronically using the X12 EDI standard. The HIPAA regulation passed in the 1990s would standardize X12 as the required data processing solution for handling medical claims.