It is pretty simple. It is called a vertical service code and there are dozens of them. They are in place to provide features from the local calling service provider. Think of them as a macro. Prior to computers being everywhere technicians used them as a form of remote access to test service functionality during and post install.

They were also used by customers to have some limited features of a company pbx (public branch exchange) without having to buy and manage a company phone system.

When my grandfather used to work for the big bell and after they were broken up, he used this frequently to redial a service number to enter additional codes for different features

https://www.nationalnanpa.com/number_resource_info/vertical_service.html